Opal

Opal is one of the few gemstones that isn't crystalline - it's an amorphous solid, a hardened silica gel containing 3-21% water by weight. It forms when silica-rich water seeps into cracks and voids in rock, then slowly evaporates over millions of years, leaving behind layers of microscopic silica spheres.

The magic of precious opal - the play-of-color that makes it flash red, green, blue, and every color between - comes from the arrangement of these spheres. When the spheres are uniform in size (around 150-400 nanometers) and stacked in orderly, three-dimensional arrays, they diffract white light into its component colors, like a natural diffraction grating. Larger spheres produce red flashes (the rarest and most valuable), while smaller spheres produce blue and violet.

Black opal from Lightning Ridge, Australia, is the most valuable variety because the dark body color provides maximum contrast for the play-of-color. The opal forms in ancient weathered sandstone, filling cracks, cavities, and even the interiors of fossils - opalised shells, bones, and even a complete opalised dinosaur skeleton have been found.

Precious opal is identified by its play-of-color - the internal flashing of spectral colors that shifts as the stone is moved. No other natural gemstone produces this effect (labradorite shows a different phenomenon called labradorescence). Common opal lacks play-of-color and appears as a simple opaque to translucent stone.

Opal is soft (5.5-6.5 Mohs), has no cleavage, and has a notably low specific gravity (2.10) - it feels light for its size. It contains water and can crack ('crazing') if it dries out too quickly. Ethiopian opal is hydrophane - it absorbs water, becoming temporarily transparent, and returns to its original appearance when dry.