I 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?