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What happened to my chalcedony?

+1 vote

used to have a piece of chalcedony that was mostly translucent, tinted a sort of light honey colour, with ever-so-slightly darker, brownish bands. It came to me labeled "Carnelian, untreated". Out of curiosity, I heat treated (read; "baked in the oven at 500*F for 90 minutes) the piece, and everything that was honey or brown had turned grey and black. 

Now, I hadn't actually expected anything to happen. I was sure I needed a specialised oven capable of reaching 900*F, but apparently not. What exactly did I do to my rock? What took place in that oven? Is it some kind of oxidisation?

asked Feb 17 by kigoet (180 points)

2 Answers

+2 votes

Heat treated rock will change color. Amethyst is heat treated to make Citrine. Real citrine is rare. Many dealers sell heat treated amethyst as citrine. It takes on a red brown hue where true citrine is pale yellow. Citrine, quartz, amethyst agate chalcedony are all varieties of SiO2. You baked your rock. Impurities may have discolored your specimen if it was not hot enough to effect the Quartz.

answered Feb 19 by Weasel (45,930 points)
I knew about the amethyst thing, I'd read about it on how to spot fake citrine (the other thing being that genuine citrine doesn't form in geodes, so it doesn't have that "tooth" extrusion).
0 votes
It can change its colour, I saw an article on this theme somewhere, if I will find, I will send it to you
answered Feb 20 by shawwesley (340 points)
Right, I just was expecting it to turn a more reddish colour, not black.