Citrine

It's quartz.
Specifically, it's quartz that's got a bit hot under the collar.

It seems that if you heat smokey quartz or amethysts then they take on a yellowish hue. the depth of the colour depends both on the colour of the crystal that you start with and the degree of heat that's applied and so the citrines that you'll find on the market range from pale yellows to deep vibrant browns. The mid range are known as 'golden citrines' and these mimic the better topazes.

Because of this, citrine has often been used as a cheaper substitute for topaz. Large citrines were set in many prized pieces from the Art Deco period between World Wars I and II, including the massive and elaborate Deco-inspired jewelry made for Hollywood stars like Greta Garbo and Joan Crawford in the 1930s. The relative cheapness of the material makes it a perfect choice for this sort of overstatement.

There are very few naturally occurring deposits of citrine and it seems likely that these to are amethysts that have been naturally heated by nearby geothermal activity. All the material that you'll find on the markets will have started off as low grade amethyst, mostly mined in Brazil, that has been heated artificially.

Here comes the scientific bit:

In my research I found out all kids of interesting things
about citrine. Here's an example:

"Citrine carries the healing properties of the sun.
It expands the auric field, filling it with light and
clearing away any lingering etheric toxins.
Citrine is also a wonderful manifestation
crystal by motivating you into taking action.
It brings optimism and cheerfulness."

Seeing as I have a large store of citrines waiting
in my stone tray, I am cheerfully optimistic that you
will buy one of the pieces featured in the thumbnails
below and I hope that it has the appropriate effect on
you when you wear it.