A: Agate is one of the more common types of semi-precious gemstones, so much so that you may not realize how many of the articles you have seen are made of some type of agate. Agate is a form of the mineral chalcedony, which consists of an aggregate of very tine quartz crystals. If it is deposited from water solutions and forms a translucent or transparent layered material, we call it agate. If it’s more uniform, less transparent, and non-layered it’s often called jasper (or for non gem types, flint and chert.) The layers may be of varying colors, and much agate is striped in attractive patterns, or has complex patterns of whorls and swirls. Carnelian and Sardonyx are types of agate, often from Brazil, but it also comes from India and other sources. A common type is Brazilian agate, which occurs often as sizable geodes, forming layered nodules. Often these are brownish tones, layered with white and gray. Slices five and six inches across are common, which are often polished as-is and sold as novelties. The stuff is also cut into bowls and decorative articles of all types. It is often dyed in other colors.

Most of the commercially sold “black onyx” on the market is actually brazilian agate, dyed black. A variety of sometimes colorful, sometimes nondescript agates are common in India, and are often cut into very inexpensive “beggar beads”. Better samples can be cut into sometimes quite spectacular and beautiful stones. Mexico produces a number of interesting agates. One of the best known types is “crazy lace”, which looks just like a lacy, crazy, pattern of mixed up whorls and swirls and lines. Its naturally usually whites and grays, but is often dyed all sorts of garish colors as well. Plume agates are less layered looking but have inclusions in them that can look like ferns, feathers, flames, or just about anything else. These are sometimes quite spectacular, and sometimes rare and expensive, for specific well known types. Montana agate is known for its basically translucent white to transparent clear colors, with suspended black or brown dots, swirls, or dendritic inclusions. It can be very pretty stuff, and is often cut into cabochons, sometimes as beads.

Lake Superior agate has a tightly layered pattern, with narrow, often quite contrasty banded layers, occurring usually in smallish nodules. Moss agate looks just like moss, often dark green, sometimes with mixed-in reds and browns, growing in the slightly cloudy to white base material. If the “moss” is dense enough as to eliminate the white, making it an opaque solid green, sometimes with red spots, we call it bloodstone, an alternate birthstone for March. The above descriptions only begin to scratch the surface of the subject. Agate occurs in hundreds, if not thousands of variations, and some entire collections are made up of nothing else. One book on the subject, called (I think) Agates of North America, by John Sinkankas, is a good start to learning about some of what’s out there. Or you can visit any decent rock and mineral shop, most have a lot of different kinds on hand.