Citrine

Citrine is yellow quartz. Its name comes from the Latin citrus, by way of the French word for lemon, and its color ranges from pale yellow to brownish orange.

Here is the industry's secret: natural citrine is rare. According to GIA and Wikipedia, most of what is sold as citrine is actually amethyst that was heated to a few hundred degrees and lost its purple. The big yellow geodes known as "cathedrals" are usually heated amethyst too. In other words, a lot of citrine is amethyst that came out of the sauna.

How can you tell? According to Wikipedia, natural citrine tends to be pale, while heated amethyst tends toward deep yellow, orange or brown. There is also an optical difference: natural citrine is weakly dichroic, meaning its shade shifts slightly with the viewing angle, and heated amethyst is not. That is hard to test at home. The stones in the photo are definitely on the pale side, but paleness alone proves nothing.

For centuries citrine was confused with topaz because of its color. According to Hebrew Wikipedia, names like "gold topaz" and "Madeira topaz" are still in use, even though the two minerals are unrelated. There is a simple way to tell them apart: topaz is harder, 8 on the Mohs scale compared with 7.

In Bolivia, nature made the combination itself: in a stone called ametrine, amethyst and citrine grow in the same crystal, half purple and half yellow. If you could not decide between the two, you can simply have both.