Chameleon diamonds is a name used to identify a class of colored diamonds (usually olive) that temporarily change color after having been stored in darkness or when gently heated. Chameleon diamonds exhibit a range of hues and tones from light to dark olive (stable color phase) through light to medium yellow (unstable color phase).
After 24-48 hours in darkness, exposure to light gradually changes the color of a chameleon diamond from the unstable yellow phase back to the stable olive phase. This is observed as an infinitely repeatable process.
A small mine of Chameleon diamonds was discovered in Southeast Asia about ten years ago. So far, most of the goods are about 20 points. The diamonds have a color shift from fancy yellow to olive green. The scientific reason for this change is unclear. To effect the change you need either light or temperature. In other words, the color shift is not like alexandrite, color change sapphire, spinel, or garnets where the effect is immediate. You must leave the stone in light for hours or heat the diamond on a hot plate for it to shift to the olive green color.
These stones tend to grade at the GIA as fancy greenish-gray-yellow or yellowish-greenish-gray. Carat size chameleons have sold for over $20,000 per carat at auction.