In response to Wjfox's most recent status update.
I decided to do some sleuthing and realized that I was the one who posted images of gemstones, but that was for a deeply off-topic thread about a story I was trying to muster the energy to write at the time.
Here's two of my favorite gemstones: the fire opal
And the rainbow opal
Actually, no, it's called a Boulder Opal
Though actual rainbow opals aren't half bad either
Another fun one has to be the bismuth fractal crystal, an artificial crystal grown from molten metal