Agates are a semi-precious gemstone. They have rings or splotches of different colors and are often used to make jewelry or ornamental artwork. The ancient Greeks believed that amulets made from agates could be worn for favor with the Gods and safety on the seas. Agates get their name from a river in Southwestern Sicily.
In order for an agate to form, there must be a host rock. Agates can form in any kind of host rock, but the most common host rock is volcanic lava rock. When the continents first began to form there layers upon layers of molten rock pushed toward the earth’s surface. The lava would go through rift zones, spewing from a volcano, or by other geological events. Inside the lava were pockets of gas. Later, as the molten rock cools these gasses escape and leave pockets.
The empty cavities and seams fill with silica. The silica is composed of suspended quartz molecules and other mineral impurities. When the silica mixture becomes supersaturated the consistency changes. It becomes a little thicker, like the consistency of a gelatin.
The next big ingredient in agate formation is time. Over time, the silica molecules begin to form miniature fibrous micro-crystals. These attach themselves to the wall of the cavity. During the process mineral impurities that have collected inside the chalecedony silica, band and form different color rings and textures.
There are still a lot of questions about the exacts of agate formation. No one has ever been able to create an agate in the lab, so many theories go unproven. Dr. Peter Heaney, Ph.D. has a theory about how the rings in agates are formed.
He believes that the silicate that precipitates on the walls has to be polymerized. This means that molecules are in long strings, as they would be in a protein. Instead, the oxygen serves as a bridge to connect two silicon atoms. In this theory the polymers are pulled out of the solution and are incorporated into crystals very quickly. Dr. Heaney believes this accounts for the rings and different shapes.
"There is a competing theory that I don’t like at all," Heaney adds, "and it’s one that Robert favors. You have a gel, a silica jello embedded in the rock, and by adding chemicals you produce periodic bands in it. You can make a silica gel in the lab very easily. You can even get the banding. But when you let the gel dry, it dries to an amorphous or non-crystalline form of silica." While high temperatures or pressures might cause the gel to crystallize, those forces do not come into play. "We know agates form close to the surface of the earth, at low pressures and temperatures," Heaney says, "and not only in volcanic rock, but in dinosaur bones."
No matter which theory you go with, there is mystic and wonder that comes with every agate.