Mineralogy Books

“And you'll be able to tell whether or not this is an opal?"
"According to the baron's instructions, it should have a transparent or white body tone, then we must look at the background color, a slight tinge of color, like a spark of fire." Stefan gave a satisfied grunt and held it up to the light. "It has a wonderful luster and a play of color."
Della peered over his shoulder. "Where do they come from? How are they made?"
"Mother Nature at her best. Unique conditions first. Heavy seasonal rains in parched desert regions where the ground is rich in silica."
"What's sillyka?"
"A colorless chemical compound, one of the most common elements on earth after oxygen."
"Then what makes this so special? You'd think we trip over them all over the place. I ain't never seen one."
"Because the conditions must be just right. Rainwater trickles down into the earth and carries silica-rich solutions into the cavities between the rocks. Then hot summers dry the earth, and as the water evaporates the silica stays in place, and over millions of years the opals form. The purity, intensity, and brilliance of color increases the deeper the rock is penetrated."
"Before it just looked like a dirty white pebble."
"You're right. The actual color is a pearl gray; sometimes you see a little pale-red or yellow tint, but with reflected light it presents all the colors of the rainbow.”
― Tea Cooper, The Woman in the Green Dress