FMF - Friends of Minerals Forum, discussion and message board

[Ken]

Trying to figure out what type of quartz this would be classified as. According to an edx analysis it's composition is Si, O, and C, according to an xrd analysis it's 97% quartz and 3% corundum and by my account it has a hardness of 9.0. What is it?

[Matt_Zukowski]

It looks like cryptocrystalline quartz. Are you sure of your hardness testing?

[Ken]

I don't consider the hardness test I've done to be 100% accurate but it's better than guessing. If you take a typical piece of quartz and scratch it using #8 or #9 from this set you can scratch it with very little pressure. I can only scratch this stone with using #9 and you have to apply a lot of pressure to barely scratch it. I'm told that the corundum is most likely causing it to be so hard. I'm just trying to figure out what mineral this would be classified as. There are a couple other stones found in that same area that appear to be just as hard. These stones appear to have been highly compressed.

[Amir Akhavan]

This is probably chert (made mostly of chalcedony).
The piece under it with the white "crust" is definitely chert. If they both came from the same spot you got your answer.
Looks a bit like pieces I've picked up along the coast in California.

About the hardness: the chemical analysis tells you it is some form of silica, so the hardness test went wrong.

[Ricky hartzog]

Kindly looks like brown common opal to me

[Tobi]

I agree with Amir, this totally looks like chert/flintstone, so I doubt that the hardness is really 9.

Tobi

[Ricky hartzog]

A corundum Crystal is 9 On the hardness

[Tobi]

Yes. But this rock has the structure of a chert, not of a corundum.

[Ricky hartzog]

I know chert, flint, agate, and jasper are all the same but different colors and chert is the white or grey version, flint is the black or grey and agate is all the pretty colors. jasper you can see thru but they are all chalcedony and from where I live we call it all flint. This is the best it's been explained to me. Is that right?

[Bob Harman]

The pictured specimen appears to be microcrystalline quartz, chalcedony. Also called, chert, or flint, depending upon color and local appearance, but the names are more or less interchangeable.
Agate is a variety of microcrystalline quartz (chalcedony) that, by definition, must show evidence of banding or lacy architectural patterns. So a larger piece helps to identify "an agate".

Corundum is a completely different mineral including a different appearance, hardness, and crystal system. Analyzed examples of both minerals show them to be completely different.
The pictured example (from only the photo - not analysis) appears to be quartz, variety chalcedony, possibly iron stained.
Bob

[Ricky hartzog]

Thanks, so more or less what I've been told. I'm just trying to learn new things. I got some real nice pieces of flint here where I live and nice quartz crystals as well. Thank you for helping clear some of the chalcedony stuff up, and I bet that piece would make a nice arrowhead. I hope this picture shows up but what would this piece be considered?

[Ken]

I too have a hard time accepting that hardness of 9.0. It may not be 9.0, but I'm sure it's above 7.0. That white substance was coating the entire stone, I had to remove it with diamond coated rotary bits. Parts of the bottom of the stone is a milky white color. Chert or chalcedony, thanks, that gives me something to look into.