Agate is a type of chalcedony quartz that forms by filling a cavity in a rock. When extracted, the layers that have formed resemble that of tree rings. Varieties of agate are formed by the differences in shape and color of these bands. The varieties in color can be anywhere in the color spectrum, with blue and green specimens showing up from time to time, albeit rare.There are many diverse types of agate found all over the world. In North America, the majority of agate produced comes from the western states of Washington, Oregon, Idaho, Montana, and California. Lace agate has been found in southern California, Mexico and Texas, and is translucent blue and white. Some moss agate is found in Yellowstone National Park and the Yellowstone River of Montana, and can be anywhere from white to green agate with green to brown to red wit black inclusions that resemble moss or landscapes dotted with trees.For centuries, agate has been used as a valuable talisman that was said to quench thirst, protect from fevers and make the wearer agreeable. Eye amulets are used to cure skin disease and ward off the "evil eye". White forms of agate were used to cure bouts of insomnia. Moss agate has been worn to help the health of one's eyes and mouth. Agate is also used to make mortars and pestles as well as the fulcrum for balances. Shooting marbles were commonly made of agate as well. Agate can be chemically and/or heat-treated to almost any color. Many of the color-banded agates on the market have been altered in some way or another.