Agate is a general description for a whole group of gems whose versatility seems unlimited.
Their popularity has spanned time since the ancients, when agate was believed to give strength and power to the wearer and later on alleged to help sleep disorders and guard against nightmares and night-demons. They are often seen in old or antique jewelry, though they are vulnerable to breaking or chipping. This term is loosely applied to any patterned quartz with curved or angular bands of color swirling within it or around it.
There is moss agate, which is a milky white kind with brown, green and black fuzzy bands trapped within it, there is banded agate and eye-agate [which only has one distinctive band within it, seeming like an eye], there is plume agate- whose bands of color have little fringes on them and looking into the stone, the agate seems to trap tiny feather plumes within it. There are many more, such as fire agate; which is perfectly clear chalcedony quartz that has a shimmering inner layer made of thin iron oxide crystals. The crystals trapped within the clear gem sparkle like fire trapped in stone. These lovely gems are mined in Mexico and Arizona , U.S.A. The problem with buying agate is that it is almost universally dyed. Most common is the black agate, which is basically dyed black and polished smooth to seem authentic.