Gregory M. Jaeger, age 15, of Florissant, Missouri, for his question:
Why are opals fragile?
The various opals are rated as precious or semi precious gem stones. They are natural minerals that somehow capture and concentrate the most beautiful colors. All of them are created by the planet Earth from common ingredients in her rocky crust. Precious opals flash fiery sparks that reflect specks of trapped moisture.
Geologists rate the lovely opal stones as hydrated silicate minerals. The silicate refers to their main ingredient, which is a compound of two of the earth's most plentiful elements oxygen and silicon. Its basic molecule is silicon dioxide, a package of one atom of silicone and two atoms of oxygen. Hydrated means that the basic ingredient is permeated with moisture. Most opals are magical mixtures of about 90 per cent silica and 10 per cent water.
This mixture gives the opal its moonbeam beauty and also its rather fragile character. By itself, silica forms the hardest of the common stones. It is the durable quartz in gritty grains of sand and in flinty hard flint stone. It is quartz, mixed with traces of various impurities that forms the hard glossy agates that come in such vivid floral colors. In these quartz minerals, the silica molecules are arranged in tight crystalline formation which is why they are hard.
In opal, the silica molecules are not in crystallized formations because they are permeated by molecules of moisture. Under certain conditions, the trapped moisture can escape. This weakens the mineral structure and also robs it of its inner glow. Sometimes an.opal loses its moisture when the surrounding air is especially dry. When heated to 100 degrees C. in a closed tube, the moisture in an opal vaporizes and departs. Hence, opals are rather fragile beauties that tend to crack.
The earth may create a batch of opals in a region of hot springs, in volcanic rocks or in sediments associated with ancient volcanic activity. The recipe calls for a high concentration of silica dissolved in chemically acid water.
The patient formation of opals occurs as the right ground water percolates through rocky crevices. Gradually its silica is deposited with moisture in jellified layers that solidify in the air. These glassy layers bend, or refract, penetrating beams of light at different angles. This is what gives the opal its inner glow. In some cases, even a pale common opal can manipulate the light to glow with fluorescence.
The basic color may be as dark as well water or as pale as milk. The quality of an opal depends upon the flashing specks of color trapped inside the glassy stone. Experts now suspect that these mini flames are sparked by infinitesimal pockets of trapped moisture. The water content that makes an opal a fragile stone also adds its beauty.