Agates are easily-recognizable stones that show the complex layering of materials, called drusy, from which they are composed. Agates come in many different appearances, from the single eye of the Mexican agate to the delicate filaments of the moss agate that suggest vegetative material in its formation. Agates are found all over the world, including the United States, Europe, Africa, Asia and South America.
What Are Agates?
What people call “agate” is actually a form of chalcedony. Chalcedony is made from microcrystalline or cryptocrystalline quartz. The typical quartz crystals structure is too small to be seen with the naked eye, however. The tiny crystals can only be seen with high-powered microscopes. Agates are found in almost every color imaginable, though green and blue shapes are rare. Layers of materials that are created as the agate is formed give the stone its characteristic multi-layered look. Agate is often fashioned into jewelry. Though it is not an expensive stone, it can create dramatic, fashionable designs. Those agates are dense, while others are hollow, demonstrating the geological and chemical forces that form them.
How Agates Form
Agates form from young volcanic rock called basalt that has bubbled up to the surface, due to the presence of carbon dioxide and water. The holes that form within the molten rock are the prime location where agate forms. The minerals within the water begin to percolate out. One of these minerals is quartz, which mixes with a type of silicon dioxide compound called moganite. Agates may contain up to 10 percent of moganite.
Internal Chemistry of Agates
If you look inside the agate under a microscope, you can see its fibrous structure. The crystalline structure begins on the wall and radiate inward like the spokes of a wheel. The first layer is fine-grained chalcedony, a mixture of quartz and moganite. Inside this is a layer of more coarse-grained quartz. The agate fibers twist in a helical fashion. How the layering effect forms is not specifically known, though water and polymerization of the silicon may play a part in the process.
Agate on the Mohs Scale
These natural forces that form agates in the ground make for a hard and very useful stone. Agates range between 7 to 9 on the Mohs Scale, a measure of the hardness of the stone and its ability to withstand scratches. For instance, a diamond is a 10 on the Mohs Scale, which indicates that it is very hard. Most agate stones measure lightly below this level, and so are not quite as resistant to scratching.