Ann Hero, age 12, of Northfield, Minn., for her question:
How are agates formed?
Rubies are red; emeralds are green, but both of these rare beauties are prized as precious gems. Agates are stippled and striped, banded and bordered with rainbow colors, and these handsome stones are treasured as semiprecious stones. Nature made many of them.
An agate is created in secrecy and total darkness, and the patient process takes ages of time. Its birthplace is a small hole, an empty pocket in a porous rock. It is formed in the earth's never ending water cyc1e and created from assorted chemicals in the earth's crust. Its main ingredient is silica, a chemical compound of silicon and oxygen.
Silica is the main ingredient in a variety of rocky m£nerals, and the silicates make up more than half of the earth's crust. They form milky white pebbles of quartz and most of the sand on the beach, a variety of glassy crystals, some clear and some tinted with rainbow colors. Semiprecious onyx and chalcedony, carnelian and jasper, chrysoprase and agate also are silicate minerals made mostly of silica.
The silicates have endured through ages of ceaseless change and upheaval in the earth's crust. They have been me1ted in volcanic lava and smashed by pounding waves, dissolved in running water and lifted high and dry with rising mountains. These geological events modeled and remodeled each deposit of silicate and gave it its present form.
The creation of an agate began when molecules of silica were dissolved by the greedy tongues of rain water lapping over the rocks. Other minerals also were stolen, and a variety of particles too small to be seen were suspended in the water. Some of the water sank through a porous bedrock and seeped into a small hole or empty pocket.
The ground water evaporated or seeped slowly away, leaving its load of chemicals behind to line the rocky pocket. The deposited agate grew in hard layers tinted with other minerals that happened to be dissolved in the ground water. Later, the bedrock crumbled to free the pocketful of agate. It was still crusted with bedrock and had to be cut open to show its handsome colors and designs.
Banded agate may be striped with blue and gray, black or ye11ow or candy colored brown. Its stripes may be straight or wavy, arid a sample of sparkling agate is ringed with colored circles. Moss agate is set with dark threads of manganese minerals that look like fairy ferns frozen in glassy globules.