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 :
Velcro
Cigarette paper
Household dust
Fabulous! Continue the journey. . . more photos can be found here.
xoxo
KP
