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Techniques for CollectorsBlue Aventurine & Blue Quartz

26th Feb 2017 16:26 UTCSophiaJoy MB

The color cause of Blue Quartz is Dumortierite, right?

What about Blue Aventurine? Some websites say the color cause of blue Aventurine is also Dumortierite, but geology.com says "Muscovite and ilmenite can produce blue, gray, or white aventurine."

So, how can I tell which is which? The hardness is about the same, isn't it?

If it is blue Aventurine, aventurescence has to be seen in the stone... Is it correct?

Thank you for your help!

27th Feb 2017 15:13 UTCDavid Von Bargen Manager

"The color cause of Blue Quartz is Dumortierite, right? no.

https://www.dmme.virginia.gov/dgmr/pdf/vamin/VAMIN_VOL27_NO02.PDF

http://www.minsocam.org/msa/collectors_corner/arc/qtzllano.htm

Cause is likely due to Rayleigh scattering (like why the sky is blue).

28th Feb 2017 23:41 UTCAlfredo Petrov Manager

Some blue quartz is blue from amphibole inclusions (commonly magnesioriebeckite or related amphibole).

1st Mar 2017 00:13 UTCLawrie Berthelsen (2)

There is a massive, opaque Blue Quartz used by lapidaries that is reportedly Quartz coloured by Dumortierite.

1st Mar 2017 16:25 UTCSophiaJoy MB

Thank you very much.

The stone I'm confused about is like the one in the photo.

I thought I would attach a photo because I've read somewhere that "blue quartz" is not the best name to use...

So, a stone like the photo, is it blue Aventurine? (the photo is not mine)

1st Mar 2017 17:22 UTCAlfredo Petrov Manager

Doesn't look like quartz at all. Perhaps marble with bands of lapis lazuli?

1st Mar 2017 17:48 UTCWayne Corwin

Kind of looks dyed to me

1st Mar 2017 18:20 UTCAlfredo Petrov Manager

Wayne, how would one go about dying parallel bands like that?

1st Mar 2017 18:42 UTCWayne Corwin

softer layers.

I've seen some marble dyed with tydybowl that looke like that.

1st Mar 2017 18:45 UTCThomas Lühr Expert

Alfredo Petrov Wrote:

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

> Wayne, how would one go about dying parallel bands

> like that?

Because of a different porosity in a banded stone (e.g. quartzite) ?

To me look both specimens dyed too.

1st Mar 2017 20:11 UTCGregg Little 🌟

I was going to comment earlier but decided to let contributors who are familiar with aventurine speak first.

I think Alfredo is right. The first picture appears to be a sedimentary rock, probably quartzite (does it fizz?). The dye shows up the fine laminations defined by clay (probably detrital), finer in the upper section and coarser in the lower section. A close look shows the gentle pinching and swelling of the layers indicative of flaser bedding. The dying in this case varies due to the varying clay content.

1st Mar 2017 21:47 UTCLawrie Berthelsen (2)

The material is marketed as Quartz, but I would describe it more as a Quartzite. The polished piece shown by SophiaJoy is the same material, just showing more banding. It is naturally coloured, I have cut many large boulders of the stuff. It has been available in Rock Shops in Australia for some years, and a Google or Ebay search will show up many examples. I understood that it was from Brazil, but it is said to be from Peru in some Ebay listings.

2nd Mar 2017 08:47 UTCErik Vercammen Expert

Quartzite is a metamorphic rock: the sedimentary precessor is first loose sand,and then sandstone. As long as the fracture (wha, you break the rock) goes around the grains, it is sandstone. When the grains break (so they are more trightly cemented), it is quartzite.

2nd Mar 2017 23:07 UTCGregg Little 🌟

Actually you can get well cemented sandstone (silica or quartz cemented) that break through the grains which is more a function of the pore spaces being cement filled. Silica cement or quartz overgrowths in a sense are bnding the grains together with the same mineral causing a mechanical interlocking. Other common cements like calcite and dolomite will also do this on the granular scale causing fractures to run through grains, but fractures will also run around grains. As Erik says for quartzite the break is through always(?) through the grains.

The transformation from sandstone is gradational and in a quartzite I would expect to see, on a granular level, a blurring of the grain contacts as the grains and cement re-crystallize to form an interlocking mosaic. Any sedimentary features would also fade and become indistinct. Bedding or foliation would be destroyed and the fracturing could be conchoidal.

There is a well known quartzite formation (approx. 2.2 billion years old) in Kilarney Park, Ontario which seems to bridge the definition. It was mined for metallurgical quartz due to its purity. Although it is very densely and very finely recrystallized, one can easily make out individual sand grains, sedimentary bedding and even ripple marks. Preserved textures of a sedimentary rock with mechanical features of a quartzite but the geological literature does describes it as a quartzite.

5th Mar 2017 08:52 UTCSophiaJoy MB

Thank you so much!

So, when I call this type of stone, I should call it "Quartzite." Is that right?

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