Opal

Opal is a delicate mineral, hydrated silica or amorphous quartz. The mineral includes a fairly large amount of water molecules, and opals should not be left in direct sunlight or high temperatures.

These two images are the same "Christmas tree" opal, rotated in different positions to show the play of colors. The stone is about 5 millimeters across, mounted in white gold as a stud earring. The whole rainbow can be seen in these scans.

Opal is a lot more common than people think, but it's usually a thin whitish film that lines fractures in rocks subjected to very mild metamorphism. Opal is commonly found with agate, which is cryptocrystalline quartz. Sometimes it is a bit thicker and has some internal structure that produces the highlights and color range of gem opal. This spectacular example is probably from Australia, where "fire opals" with dark backgrounds are mined. The one below is definitely a fire opal.

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