Polly Bassett, age 12, of Milwaukee, Wisconsin, for her question:
What causes the different forms of quartz?
Onyx and opal, agate and amethyst, flint and a large assortment of other stones are merely different forms of quartz. The basic mineral in all of them is made of the same molecules. Geological events arrange these molecules in different patterns and other minerals add the color variation.
The mineral quartz is a compound of two very plentiful elements oxygen and silicon. Each of its particles is composed of two atoms of oxygen and one atom of silicon. It is called a silicate mineral and, in one form or another, quartz makes up one twelfth of all the material in the earth's crust. Its molecules were formed when the earth was born and they have been through many adventures. Some have endured the heat and pressure of volcanic activity. Some have been dissolved and deposited by water. Some of these igneous and sedimentary forms of quartz have been remade into metamorphic rocks such as durable quartzite.
Particles of dissolved quartz tend to settle and arrange. themselves in crystals. The shapes of the molecules may cause the crystal to grow in a six sided shaft with pointed ends. The formation depends upon temperature. In springs below 573 degrees Centigrade, the crystal grows only one side. More heat is needed to form full crystals. Geologists can tell the temperature of the rock in which a piece of quartz formed from the shape of its crystal.
As the crystal grows, traces of other minerals may be mixed among the molecules of silica. Iron oxide or manganese may tint the glassy crystal pink or rose or violet amethyst. Scraps of decayed material may add smoky grays or rich yellows. Some crystals get a rosy blush from fragments of titanium. If no impurities at all are in the crystallizing mixture, the quartz crystal is glassy clear.
A large assortment of other quartz rocks are massed together from microscopic fragments. These are the waxy chalcedonies made from tiny crystals arranged in parallel fibers. A trace of nickel oxide makes apple green chrysoprase and a trace of hematite changes the same mineral into deep, rich red jasper. Other impurities add the colored round rings of agate and the ribbon stripes of onyx. Hydrocarbon from ancient diatoms add the blacks and the dusky browns to pebbles of flint.
When water molecules are trapped among the settling molecules of quartz, the mixture may form pearly opal. Traces of other minerals in the mixture add the moonglow of a precious opal, the glowing flame colors of a fire opal or the wild dancing colors of a harlequin opal. Most opals are 10% water and waxy white common opal is water and plain silica.
Molten quartz pours out with rivers of lava. Some of it forms massive layers sometimes veined with gold: When the temperature is high, the quartz is much coarser than the fine grained stones that form as watery sediments. Coarse grains of durable quartz may become embedded in granites. When the softer minerals around them wear away, they are freed to become gritty little grains of golden sand.