Common Opal
Precious Opal from Australia
More Opal in rhyolite from
Australia.
CHEMISTRY - SiO2 .nH2O
CRYSTALLOGRAPHY - NONE, Opal is amorphous
CRYSTAL GROWTH AND HABITS - Opal is massive, often botryoidal or stalactitic.
COLOR AND OTHER OPTICAL PROPERTIES -
HARDNESS - 5 - 6
SPECIFIC GRAVITY - 2 - 2.5
BREAKABILITY - Conchoidal fracture and brittle
PRECIOUS OPAL is transparent to translucent with an internal play of colors. The production of color in precious opal gives the material it "fire". The color production comes from the interference of light that is produced by the microscopically small spheres that make up the mineral. It is similar to the color that is produced on the surface of CD's (which have finely spaced grooves) or the color produced on the carapace of some insects.
COMMON OPAL is commonly milky white to slightly yellowish, brownish or greenish. It is translucent and often has a waxy luster.
HYALITE is a clear variety of Opal that forms globules or clusters of globules becoming botryoidal.
GEYSERITE is the term given to the opalescent deposits of silica that accumulate around hot springs and geysers, especially those at Yellowstone National Park. The material is gray and has an earthy luster.
DIATOMITE is the rock name given to coherent lumps of diatomaceous earth. This sedimentary rock forms as a result of compaction of siliceous (opaline) diatoms. It looks very much like white chalk.