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Techniques for CollectorsHydrogrossular garnet+Idocrase Cabochons

20th Jan 2013 14:23 UTCram sati

I have many hydrogrossular garnet and Idocrase stones cut into cabochons(they are a mix of hydrogrossular garnet and idocrase I believe) from west sumatra;the people who carves these stones in indonesia allways says that if the cabs are drown into rainy water in a pot for several months become more cristal,more clean inside the stones;I got these cabs I attach and after a couple of months inside water they looks like the chalky appearance get a little dissappeared,they looks like more cristal,transparent...it is difficult to explain but when I look at them I see a change

People carve this in Indonesia allways says rainy water that flows and put the stones inside many months...

Does this have a scientific explanation?could be that the inclusions I see like chalky inside are limestones or serpentine or similar that once inside water dissolves in the water so it comes out from stones by dissolving into the water?

Best resgards

Alex

20th Jan 2013 17:12 UTCOwen Lewis

Mixed minerals may have a porous structure that will absorb water. This porosity (empty space in the rock can be shown to be >5% of the rock's volume in some cases. Looking at your pics, I'll guess that this is what is probably happening here.

If you have an accurate and calibrated balance or know someone who has, you can test this as follows:

- Take a few samples of the clean rough stone. Bake in an oven for a couple of hours at about 150 deg C. This should be sufficient to evaporate and drive out any resident moisture lurking in the pores in the pieces.

- Remove, a piece from the oven. Weigh quickly in air, Record the weight (A).

- Whilst the stone is still uncomfortable to handle with bare hands, place it in a dish of clean water (treated with just a couple of drops) of wetting agent so that the piece is well covered.

- Repeat for all other samples. Leave all at room temperature for to soak for two days.

- Remove a piece from under the water and weigh using a hydrostatic weighing kit to establish the weight when suspended in water (B).

- Quickly remove loose water from the outside with an absorbent microfibre cloth and re-weigh in air as quickly as is consistent with accuracy (C).

- Repeat for all other test pieces.

Using this set of recorded weighings, for each piece, calculate the following:

- A/(A-B) = SG of specimen stone

- C-A = wt of water drawn into the stone.

Knowing the SG of each specimen and that the SG of water is 1.000, it's then simple to calculate further the volume within the stone that has been filled by water and to express this as a percentage of the volume of the entire piece.

This basic experiment may not allow you to calculate perfectly the % porosity of your samples but the true porosity cannot be any *less* that the percentages you calculate using this method.

If your samples show significant porosity, then you have a proved answer as to why the 'folk-lore' wisdom may, indeed, be correct:-)

20th Jan 2013 17:27 UTCram sati

Thanks very much Owen Lewis

Very good explanation to calculate the Specific Gravity and to calculate in some way an average the porosity

So I see it more cristal or clean as the water penetrated the porus;but once the water evaporate again it will be bcak to normal apperance the stone like before?

Best wishes

20th Jan 2013 19:05 UTCOwen Lewis

The water that replaces air has a substantially higher refractive index. In an optical sense only, this seems to 'heal' (but only partly) the minute fissures in the specimen, causing them to reflect less light and thus to seem clearer. Bake again in the oven to drive out most of the water and you will have a specimen that will once again seem less transparent and more translucent than when the fissures (pores) were largely water-filled. Use a clear liquid of a higher RI that water to fill the fissures and the pieces should seem even clearer then it did when the fissures were filled with water. This is exactly why the fissures that are commonly found in good emeralds (RI around 1.583 - 1.577) are routinely filled with an oil that has an RI of 1.515 - it make the flaws in the stones much less noticeable.

For the avoidance of doubt.... the method described in my previous post, does not determine the *average* porosity by %age of volume but, rather, establish a minimum porosity which each sample has been proved to have. With a first-class balance and good working practices, I'd expect this proven minimum not to be too far the absolute maximum porosity (say within 1% of the total specimen volume). To get closer to perfection, it would be necessary to do the saturation of the freshly-baked stones in a vacuum chamber.

Warning. Don't be tempted to improve the clarity of stones by saturating them with a fluid of a higher RI than around 1.515 (cedarwood oil) as these all have problems associated with them that include toxicity (some seriously so) and discolouration over time.

20th Jan 2013 19:39 UTCram sati

Again Owen Lewis thanks very much

Very good explanation

Best wishes

Alex

21st Jan 2013 00:49 UTCOwen Lewis

Glad if it helps, Alex.

Best wishes,

Owen

21st Jan 2013 12:15 UTCRock Currier Expert

If your material looses water over time and becomes less opaque, it will not be received well in the gemstone market. There are a number of treatments where the water can be replaced with other materials so as to increase the attractiveness and stability of the stones. This is done to a number of "gem materials", perhaps turquoise being one of the most commonly treated stones. It can be something as simple as placing the stone(s) in hot Paraffin fir a while and then removing them and wiping off the excess Paraffin to putting the stones in a pressure chamber and treating them with plastic under heat and pressure. If these stones do dehydrate and become less attractive, probably some sort of treatment could render them to be commercial, but probably not of any great value.

25th Jan 2013 19:42 UTCram sati

Thanks for the comments and help

These are same material but more transparent than other ones.

I have seen Californite from Happy Camp Jade in california and it look likes the same

Californite is hydrogrossular mix with idocrase?

Best wishes

25th Jan 2013 22:20 UTCOwen Lewis

Yes, that mix is Californite.

25th Jan 2013 22:29 UTCRock Currier Expert

Thats pretty material, it should have some value as a gem stone. Probably all it needs is to be given a fancy name and a lot of marketing.

26th Jan 2013 12:36 UTCram sati

Do you think the more transparent the material the more proportion Idocrase and the more opaque more hydrogrossular garnet?

How they mix both minerals in the same rock?

If Idocrase is Ca10(Mg, Fe)2Al4(SiO4)5(Si2O7)2(OH,F)4 and hydrogrossular garnet : Ca3Al2(SiO4)3-x(OH)4x

Each one has a different crystal structure,hydrogrossular has a unitary crystal belongs to Cubic system and idocrase tetragonal;they intertwine both unitary cells or how does it do?

Sorry I am not very much know about it

Regards

29th Jan 2013 02:53 UTCOwen Lewis

ram sati Wrote:

-------------------------------------------------------

> Do you think the more transparent the material the

> more proportion Idocrase and the more opaque more

> hydrogrossular garnet?

I don't think so. necessarily. Vesuvianite can range from transparent to opaque and hydrogrossular from translucent to opaque so two situations are possible with a range of variation between:

1. The specimen seems perfectly transparent. In this case it must either be vesuvianite only or with so little hydrogrossular in the mix as not noticeably to affect the transparency.

2. The specimen is somewhere between translucent and totally opaque. In which case, the proportion of each mineral can't be estimated from the appearance of the mix and proper testing must be used.

> How they mix both minerals in the same rock?

A very good question and one which a good crystallographer might be able to answer. My basic knowledge of crystallography only takes me as far as:

- A cube is actually only a special case of a tetragon in which not only are all three axes at right angles to each other but all three axes are of equal length as opposed to only two being the same length in a tetragonal crystal.

- It follows that it should be possible for a mixed lattice to form, if the unit cell dimensions of each are sufficiently close to each other for the strength of the bonds in the lattice to take the strain caused by small differences in the unit celll dimensions.

This may be so in a vesuvianite/hydrogrossular mix, because the birefringence of vesuvianite can be as low as 0.001. In other words its unit cells are very close to the special dimensions of a cubic system cell. But then the cell volumes either have to be very similar also - or to in some simple mathematical ratio to each other each other.

Maybe a crystallographer might like either to correct or dot the i's and cross the t's on this?

> If Idocrase is Ca10(Mg,

> Fe)2Al4(SiO4)5(Si2O7)2(OH,F)4 and hydrogrossular

> garnet : Ca3Al2(SiO4)3-x(OH)4x

> Each one has a different crystal

> structure,hydrogrossular has a unitary crystal

> belongs to Cubic system and idocrase

> tetragonal;they intertwine both unitary cells or

> how does it do?

> Sorry I am not very much know about it

Hey Alex, you understand enough to see the problem:)-D. Let's hope that you can receive a more complete answer than I can provide.

Regards,

Owen

22nd Oct 2013 03:58 UTCram sati

I tested some samples;an x-ray difraction and the results is that some material,the more translucent is vesuvianite and also in it's composition there is chlinoclorum,some other sample results vesuvianite and grossular garnet and also chlinoclorum and some other a yellow opaque whitish one quartz with albite.

19th Oct 2015 23:23 UTCRich Kelly

I mine Californite and I do get pure text book Idocrase and pure text book grossular garnet both have been tested many times. My best pieces are the purest. They are different minerals Californite is a local variety of Idocrase

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