Opal is a birthstone for October. (Time to get a birthday present, maybe?)
Technically, opal is not a mineral, but is referred to as a mineraloid because it does not have an ordered internal structure. Instead atoms in a regularly repeating lattice, opal is made up of spheres of silicon dioxide. This does not mean it isn't valued and beautiful in jewelry!
The play of color in precious opal comes from the refraction of light from the spheres. Precious opal has play of color and these stones can be classified by the color of the background: black, white, and crystal (clear) are the most common terms. Fire opals rarely have play of color but are a stunning orange color. Other desirable opals that lack play of color can be green, rose, or yellow.