Oh, the mystique of opal! Chemically, opal is hydrated silica, similar to quartz. And it can contain up to 21% water. Water! Opal, being amorphous, is not truly a mineral but a mineraloid. One of the scientifically accepted standards defining a mineral is that a mineral must have a crystal structure, which opal lacks.
There are so many types of opal, each one delightful, but I'm focusing on Mexican Boulder Fire Opal. "Boulder Opal" is a term used for a rough or a cut gemstone that displays opal within its surrounding rock matrix. "Fire Opal" is so called due to the colors it can be found in, rather than the fact that there is any "fire" within the stone.
Since opal is known to form near the surface of the earth at relatively low temperatures where there has been volcanic activity, this silica material fills in cracks and seams in matrix rocks. A good example of fire opal found in volcanic matrix comes from Mexico, sometimes called "jelly" opal. These stones do not usually show any play of color but a magical clarity and have a watery effect.
I have a few versions of this amazing stone in my shop:
Now for a little side trip. In the 1960's, the reason for opal's color play was discovered with the aid of the electron microscope. Opal is composed of tiny silica spheres that can be arranged in an orderly pattern. This diffracts the light entering the stone into the spectral colors. A light wave diffracted through the opal causes a color sheen or scintillation in the stone. The density and pattern of the aligned silica spheres are responsible for the different colors refracted in the opal. Here is common opal under an electron microscope:
Another interesting thing made of silica is the needle in the mouth of a female mosquito. This lead me to looking up female mosquito mouths:
Of course, I'm a 'sucker' for tiny things. Even teeny, teeny, TINY things. Have a look at these goodies :
Fabulous! Continue the journey. . . more photos can be found here.