Anyway, I dug it out to take some pictures and thought how the yellow quartz looked remarkably like citrine and wondered if it might indeed be so. Going by the maps on Mindat I was astonished to find that not only was citrine to be found in England but of the very few recorded sites, two were within spitting distance of the location in question!
I wonder if any quartz experts on Mindat might take a look and confirm or otherwise my guess (and it is only a guess) that I may have citrine. The vug is on a medium sized piece of hematite and the enclosed picture has not been enhanced in any way at all....it is exactly 'as is' other than reduced in file size to comply with Mindat rules.
Thank you,
Regards,
Jay.
Can you see if there is a broken crystal with the yellow color IN the crystal? If that is the case, citrine is a possibility, but for me iit looks more like a crust OVER the crystals (but the color is nice!). 
To my eye looks like staining as well, but a pretty specimen nonetheless. This is the kind of appearance seen all the time in Indiana geodes. I literally have hundreds of examples of Indiana quartz geodes with secondary iron staining in all shades of yellow, orange, red and brick red. On some the coloration is clearly confined to the surface of the quartz crystals while in other examples the coloration has infused into the quartz crystals.
Your example shows most if not all the coloration confined to the surface and between the crowded tips. The bases of your crystal tips is largely whitish.
This is all secondary iron staining so I prefer to call these quartz crystal tips "citrine colored" quartz rather than true citrine. True citrine, to me, implies not only the quartz color, but a uniform crystal with a specific mode of formation and origin. If the term citrine just involves the crystal color and NOTHING else than maybe I might change my mind, but I think true citrine involves more than just quartz color. Scroll thru "Midwest geode" thread for examples and discussion of this common iron staining phenomenon. CHEERS…….BOB Here are some iron stained Indiana geode examples. Are the pinkish examples "rose quartz"…….I think not. Are the yellowish examples "citrine"…….I think not. Are the other shades something else……..I think not. They are just all shades of iron stained quartz. Hope this helps and CHEERS…..BOB
Many thanks folks for your responses, particularly Bob with his detail and his images, very nice!
I have to say, I wouldn't have made this particular query had I not seen that citrine was to be found in the locality. I am well aware of iron stained quartz, Cornwall is heaving with the stuff, it's just that this particular piece is so yellow, amber/gold yellow.
In due course I shall be showing it to the few Mindat members that I actually know, not to dispute anything, just so as they can see how nice it is in the flesh.
Many thanks again chaps.....your help is as ever much appreciated.
Regards,
Jay. 
FWIW Jay, it looks like citrine to me. But from the pic, that's just a guess. Go over it carefully with a loupe/microscope with light spot delivery from something like a 2-3mm flexible fibreoptic feed. Look for increase in colour saturation in re-entrant angles. If you find that, it's surface iron staining. You'd then need to treat a piece with an effective iron stain remover, assessing the colour again after removing the surface staining.
Given the circumstances of the find, *some* surface staining would be pretty likely, would you say?
Failing all else, break up a single yellow crystal and rotate the larger fragments under your microscope against a pure matt white background. with diffused incident lighting. Jay
The crystals at the top show that it's a coating, iron stain, and you can see it iridescent flash off the iron coating in the less well lit center of the photo,,, and BTW ... nice clear photo (tu)
If you have a loose crystal, you could try SIO (Super Iron Out) or oxalic acid on the crystal to see if its removable.
Tho some coatings stick good.
A broken crystal, cross section, is usually the easyest way to tell.
Nice Pocket ! Hi Owen, thanks for your input. As you know, my workstation permits only camera work or 'scope work and at the moment it is in camera work mode ;-)
Once the Meiji is back in place I shall do as you say. Yes, iron stained quartz it to be found at this location but I have never seen it looking anything like this, not even from other locations. This really is as clear as ice!
As far as I can see myself, via an illuminated loupe, the broken crystals are yellow throughout and not just on the surface so am beginning to wonder now!
Wayne, thanks also. I did wonder what was creating the iridescence.....now I know! As you mention, iron stain remover might be able to resolve this and has in fact been on my shopping list for a while....I just need to get out more often :-S
Thanks also for the picture compliment Wayne. As it happens, the chap listed above you on this forum topic has had a large part to play in the improvement of my mineral photography......when I look back to some of my earlier efforts I almost shudder! Many thanks Owen :)-D
Right....more siderites to photograph....
Many thanks again chaps....
Regards,
Jay. 
Looks very much like iron-stained/included crystals.
There's a simple test: citrine is dichroic.
Unfortunately, the dichroism is rather weak and you need to break off a crystal and rotate it in front of a polarizing light source (LCD display). Citrine behaves the same as smoky quartz in that it gets darker at the same orientation as the smoky quartz crystals (compare image on the smoky quartz page).
Ferruginous quartz and heated amethyst don't show dichroism, lab-grown iron-stained crystals show dichroism, but they are darkest at a different orientation than citrine (can't remember if it was parallel or perpendicular to the lights polarization plane). Amir,
The method you describe is really not the best and can miss easily miss weak pleochroism. With a proper instrument, dichroism should be detectable in all coloured both is present transparent coloured quartz except for aventurine and praseolite. The dichroism varies from very weak to strong in effect according to variety and also from specimen to specimen.
The tool for checking this is a dichroscope. Also no power requirement other than daylight:-) The better design uses a couple of calcite rhombs fixed in a 2in metal tube. This enables both of the colours of a uniaxial crystal to be seen side by side, which makes spotting a weak change much more reliable.
Di- or trichroism is a different phenomenon from the light extinction between crossed polars that creates isogyres in viewing all doubly refractive crystals when correctly orientated. It is as an emergency polariscope that some people use a laptop screen and one lens of a pair polaroid specs. A decent dichroscope only costs around GBP 30 and in my view should be owned by all who peer at transparent, coloured and doubly refractive crystals.
As discussed on Mindat before, much (most?) 'natural' citrine that is sold is formed by heating natural amethyst (there not being enough natural citrine to meet demand). And that's before we get onto synthetic production. Since citrine is typically weak to moderately dichroic, it follows that the pleochroic characteristic of amethyst is not destroyed by heating (to 550 deg C anyway) Heated to above 600 deg C, citrine turns either colourless or milky and, yes, the dichroism will then disappear, No colourless, opaque or singly refractive crystal is ever pleochroic. And nor is a uniaxial crystal viewed down the C axis.
Do try a dichroscope, I seriously recommend it.
@Jay. Glad to have been of a little help (yes, that is a pretty and well executed pic!). As we have discussed 'Those that can, do. Those that can't, teach. And those that can't teach, lecture' :)-D The method works for people who do not own a "proper instrument".
Heated amethyst does not show dichroism at all. Depends on the temperature heated to and the sensitivity of the method used for observing pleochroism. I say again, with heating, amethyst turns into citrine and citrine to something colourless before it loses dichroism. Depending on the exact composition the dichroism of amethyst varies in the typical range weak to moderate. This is not a personal opinion but an observation that many share - and is found in the several reference texts of those who study colour in crystals. My guess is that the problem you have is not a lack of interest/effort but of an inadequate method.
There's really no more to say than please get a calcite-based dichroscope and try again - being careful to avoid presenting the c axis of quartz to the dichroscope aperture. Start using it with minerals that show strong pleochroism and in all three colours (such as andalusite, tanzanite etc.). Once you can find all three colours, work down to the more difficult to discern stuff like unixial amethyst and citrine than may show two only slightly different shades of essentially the same colour. If you want to start with a uniaxial mineral variety, well saturated emerald (especially Colombian) is a good place to start. Owen, what causes dichroism in amethyst are the physical properties of the color centers, not the quartz itself. Dichroism is not inherited between different causes of color by treatment of any kind. If the amethyst color goes, the dichroism goes with it. If the treatment causes the amethyst to assume a different color, it may or not show dichroism, depending on the physical nature of the newly formed color agent/center.
I got a number of different citrines, and they all show dichroism with the method described above, or using a polarizing filter. It is very obvious, just as with amethyst, prasiolite, smoky, pink and rose quartz. It is also obvious for lab grown yellow quartz (from hydrothermal synthesis), which very obviously is different in behaviour (different axis) from citrine. But even intensly colored heated amethyst does not show it. The dichroism is either absent or below the detection limit of that method. Given the intensity of the colors of the samples compared (citrine vs. heated amethyst), the nature of the color agent must be different.
Amethyst is slightly biaxial, btw. 
If you look closely at the crystal with a 10x loupe or with a microscope you can usually determine if the crystals are citrine or not. If they are iron stained, the iron staining is on the surface of the crystal or in a layer just under the surface. Close examination should reveal if the color is iron staining or if the color is the color of the entire crystal. Jay, I believe that a visual ID can be made for citrine, and this is not citrine.
Remember that citrine require two things, radiation and heat. While the radiation is more or less constant geologically, the heat may not be. Your Quartz cluster doesn't show the effect of radiation, which would turn more / most / all of the xtls to amethyst. This small geode would not have been exposed to radiation in such a narrow localized zone.
Small citrine Quartz clusters are more likely to be like the example shown below, from the mindat archives. This example shows the effect of constant radiation and temporary heating, which turned older material yellow, while newer material is amethyst. Close examination may show the surface of these crystals to be clear, as the newest material has not yet had time to react to the radiation.
http://www.mindat.org/photo-361296.html Hi Amir,
Amir C. Akhavan Wrote:
-------------------------------------------------------
> Owen, what causes dichroism in amethyst are the
> physical properties of the color centers, not the
> quartz itself.
That is not my understanding. Rather as unpolarised light enters an anisotropic and transparent crystal the light is (1) polarised, the one part at 90 deg to the other. These are called the ordinary and the extraordinary rays. (2) the velocity of both rays is reduced but differently. They may also become 'coloured' by some selectively absorptive effect. The and rays travel at different velocities (witnessed by the fact that an anisotropic crystal shows two refractive indices, one for the . and one for the ray. Whether the ray travels faster or slower than the ray is said to be a part of that crystal's optical character, and can be recorded according to convention as + or - after a capital letter. All uniaxial crystals are denoted by the letter U. So, all the transparent quartz varieties have the optic character U+, as the ray travels faster through quartz than the ray.
Where, by selective absorptive effect, the two rays become coloured on entering the crystal their colours show differently in ratio to the difference in their velocities. The colour difference can appear in one of two ways. (1) Two strikingly different colours or (2) Two more or less subtly different shades of the same colour.
Determining the optical properties of transparent crystals st the most important single method for identifing a transparent mineral where all evidence of locality, external mineral associations and crystal form have been removed - that and examination of its inclusions under a suitable microscope. All this - - and much more - is set out with clarity in 'Crystals & Light' an excellent little book by Elizabeth A. Wood and all other standard texts that deal with the subject.
Some pleochroism is strong enough to be observed without any tools at all. Such crystals can be seen to completely change their colour when back lit and rotated to all angles in the hand. Andalusite is one example. Sapphire *may* be another. Here is dichroism in a piece of Australian sapphire rough, captured simply by turning the crystal through 90 deg in front of the camera when back-lit with white light. The only change in the setup is the orientation of the specimen.
Now, your suggested use of an LCD screen:
1. Light emitted from the screen is already polarised in one plane.This means that for any relative orientation of the screen and crystal, only the colour of the or the ray can be observed, which it is is determined by the orientation of the screen relative to the stone, since both rays cannot be exist at the same time where the input light is already plane-polarised. You can only see one or the other ray at any one time. Otherwise, an polarising LCD screen used s a backlight neither helps nor hinders the observation of pleochroism more than would any white backlighting of a crystal under test.
2. Where the pleochroic effect is weak, the eye only detects the subtle colour change where the two colours are presented to it simultaneously. Your method cannot do this and you have to rely on your memory of the one shade of colour whilst looking at another. It's simply no way to do the business, giving little/no improvement over what pleochroism could be observed without the use of such a screen.
> Dichroism is not inherited between
> different causes of color by treatment of any
> kind. If the amethyst color goes, the dichroism
> goes with it.
Not so, if another colour is caused, as in the transmutation of amethyst to citrine. the dichroic effect is still present, and the pleochroic colours change. Only where there is a transmutation from coloured to colourless - or there is a loss of transparency - is the pleochroic effect destroyed.
> If the treatment causes the amethyst
> to assume a different color, it may or not show
> dichroism, depending on the physical nature of the
> newly formed color agent/center.
I think that is a misunderstanding of effects of different processes, all working together. If you can give me details of a standard reference test that says as you describe, I shall certainly read it with interest.
Whilst dichroscopy is a very useful technique, results obtained can vary somewhat not only from specimen to specimen but from one observer to another and there are pitfalls. For example, the strength of colour(s) a crystal presents will vary according to the length of the light path through a given crystal. This can easily be more than doubled by rotating a crystal fragment in the line of view. Also pleochroism is only one of several colour-changing effects. Two or even more effects may occur in one and the same specimen simultaneously. Sorting out what change is caused by which mechanism requires patience and some care and expertise. Thus, whilst it is easy to set out fairly simply what the underlying rules are, what is actually observed and the reasons for a particular observation can vary. In terms of the determination of a crystal's identity, pleochroic effect is only of key importance with a few identifications (e.g. andalusite (species) or tanzanite (variety of a species). In the majority of cases it only gives (or fails to give) supporting evidence to a determination of identity carried out primarily by different means altogether.
IMHO, the only solid tests for the differentiation of surface yellow-stained quartz from citrine are (1) careful surface examination preferably under magnification, or (2) shattering a crystal to examine the colour of a splinter from its interior. Am reading all of this but am way out of my depth :-S
My workstation has now reverted back to 'scope work and am trying to hunt down the piece that I opened this discussion with so as to check it again under magnification.....not having a lot of luck thus far....too many rocks in too small a house!
Regards,
Jay. OK gentlemen....I have now found the piece and had it under the 'scope.
Virtually all of the broken yellow crystals are indeed yellow throughout so I think that knocks surface staining on the head.
Interestingly, of all the eight locations listed on Mindat for citrine for the entire UK, two of these locations are within spitting distance of Wheal Drea Mine. Also, on the Mindat Citrine page it mentions that when heated, amethyst will turn into 'fake citrine' of the type often sold in gift shops. As amethyst is also to be found at this location, does it not then follow that due to natural hydrothermal activity my piece could indeed be genuine?
Am off to visit my mineral mentor next week so shall take it with me. If he is unable to help then I shall be contacting a certain Mindat member (who is a gem specialist) offline to ask if I might post it to him for his perusal.
I'll get there in the end.....famous last words!
Regards and thanks again everyone.
Jay. Jay I G Roland Wrote:
-------------------------------------------------------
> OK gentlemen....I have now found the piece and had
> it under the 'scope.
>
> Virtually all of the broken yellow crystals are
> indeed yellow throughout so I think that knocks
> surface staining on the head.
You may of course have both. I think it likely that you do :-)
>
> ... Also, on the Mindat Citrine page
> it mentions that when heated, amethyst will turn
> into 'fake citrine' of the type often sold in gift
> shops. As amethyst is also to be found at this
> location, does it not then follow that due to
> natural hydrothermal activity my piece could
> indeed be genuine?
You have the essential point I think, Jay. The naturally coloured and the coloured by heat treatment after mining citrine crystals cannot reliably be told apart. Neither is fake; both are equally real. The only way to be sure a citrine crystal was coloured by geological process alone is to have proof absolute of place of origin and handling thereafter - as you do for yours.
BUT if that was the whole story, how does ametrine form and why is it so rare in nature? I'll e-mail you a good article on that by Prof Rossman (he of Rossmanite) and a couple of the Russian guys who did the research for its successful synthesisation.
>
> Am off to visit my mineral mentor next week so
> shall take it with me. If he is unable to help
> then I shall be contacting a certain Mindat member
> (who is a gem specialist) offline to ask if I
> might post it to him for his perusal.
>
> I'll get there in the end.....famous last words!
Sure you will! JAY I will repeat: your example is NOT true citrine. Citrine refers not only to a yellowish color variety of quartz, but more than just the color. Many iron stained examples have the yellowish tint both on the crystal surface and infused into the crystals. Yours, apparently from what you say, has both a surface staining and some infusion. The iridescence also supports the iron staining. Also the vug environment with the yellowish quartz crystal tips supports an iron stained example rather than true citrine. So I would call your example something like "citrine colored iron stained quartz". Nice example, but NOT TRUE CITRINE! CHEERS……BOB Owen, do you mean to say that the "selectively absorptive effect" is a property of the crystal?
So if I paint a crystal with a color of my choice, it will show dichroism?
It will not.
And a colorless quartz crystal would then show dichroism in white polarized light.
Which is doesn't.
So I don't think this is what you meant to say. It would be simply too stupid.
Dichroism is a property of the color centers, and of course their properties reflect the physics of the atomic lattice.
That is why a lab-grown quartz which is yellow because of iron can show a dichroism that is opposite to that of an equally yellow citrine (darkest color with polarized light parallel vs perpendicular to the c axis).
Color agents that are built in the lattice in a regular manner (like color centers) will reflect the symmetry properties of the atomic grid and its electric fields. So quartz crystals with color centers based on substitutional atoms (Al,Fe,P in place of Si) all show dichroism. Heated amethyst is colored by iron oxides that are randomly oriented with respect to the crystal lattice, the iron is essentially exsolved from the lattice, hence there is no dichroism.
> 1. Light emitted from the screen is already polarised in one plane.This means that for any relative orientation of the screen and crystal, only the colour of the or the ray can be observed, which it is is determined by the orientation of the screen relative to the stone, since both rays cannot be exist at the same time where the input light is already plane-polarised.
Owen, this is wrong.
What happens when a light ray enters a crystal? What happens when you rotate the crystal or polarization? Why does a rock crystal not get dark or bright when I rotate it in front of the screen? Is there any difference in appearence to the naked eye when compared to what the crystal looks in randomly polarized light? No. They look the same. You need a second polarizer between the crystal and the eye to notice any difference. Why is that? But you do not need a second polarizer behind the crysal to see dichroism (it will instead cause "unwanted" effects). And why is that?
I recommend you to read a textbook on optical crystallography, or even just a mineralogy textbook will probably do.
> 2. Where the pleochroic effect is weak, the eye only detects the subtle colour change where the two colours are presented to it simultaneously. Your method cannot do this and you have to rely on your memory of the one shade of colour whilst looking at another. It's simply no way to do the business, giving little/no improvement over what pleochroism could be observed without the use of such a screen.
At a giant cucumber contest:
"And here we have the winner, Linda, with a 3 ft cucumber!"
(Applause)
"Number two, Fred, whose cucumber grew to 2 ft!"
(Applause)
Fred: "But my cucumber is larger! You missed the subtle details, you must measure it in millimeters!"
What I said is that dichroism in citrine is weak, but it is easily noticed in front of a LCD screen or with a polarizer filter before or behind it. Nothing can be seen with this method using heated amethyst. So, as already explained above, there is a substantial difference between the two materials, enough to be of diagnostic value.
If someone else demonstrates that there is pleochroism of an order of magnitudes smaller with a sensitive method that doesn't mean anything with respect to the diagnostic value of the method above.
You are essentialy saying "but you ave not checked if there is a tiny micro minute dichroism!"
I say "So what? I can see it in amethyst, I can't see it in heated amethyst, and I can see it in citrine. That is a substantial difference."
Take a look at the photo on the smoky quartz page showing dichroism.
Have I found a way to transfer the biased interpretations of what I halluzinated into my memory to that photo?
I need to do the equivalent for citrine and put it on that page. Jay
Pry off one of the crystals along the edge,, then bust it into pieces and see if the inside is truely colored :-S Amir,
I think we should discontinue as this exchange has become unproductive. I repond this final time since since error in in the understanding of and wrong explanation of scientific methods should not pass unchallenged for misinformation is decidedly unhelpful.
1, Pleochroic effect is not caused by 'colour centres'. Since it is an effect observed in a whole range of coloured crystals and some other materials that have *no* colour centre(s). it follows that your assertion must be incorrect and that the true cause is something else. Further, you were asked to provide an authoritative reference for your assertion and have not done so.
2, I am sorry than my brief explanation of the cause of pleochroism failed to be understood. A full explanation would be burdensome and recipients need need a reasonable pre-knowledge of the properties of light. For a true understanding of the pleochroic effect and its colour changes, any may read 'The Physics and Chemistry of Color' Nassau.K. pp 29, 90-91, 93, 96, 98, 334 and 355. For an understanding of the causes of colour in the gem minerals, read and digest the whole book. There is no better source that has the same scope. Last revised in 2001, it remains the standard work.
I'm happy you can show the appearance of some pleochroic effect by use of a laptop, camera, stand and a combination of images. My only point is that a calcite rhomb dichroscope is much cheaper, infinitely less cumbersome and is easy to use anywhere, anytime. It is also far more effective in most cases :-) This confirms that you confuse two different things:
Pleochroism and Birefringence.
Hence the odd conclusions. Oh dear, where has this thread gone?
All I asked was whether I had stumbled upon citrine or no :-S
