Agate is chalcedony quartz which has concentric layers and is found in a range of colors and textures. Each stone is formed by the filling of cavity in host rock and because of this it is found as round nodule with concentric layers like the rings of a tree trunk. They are mainly associated with volcanic rocks but can be seen in certain metamorphic rocks as well. The main features of this gemstone are its fineness of grain and the brightness of color. Many specimens are hollow because the deposition is not enough to fill the cavity. If the deposition is not fully filled, then the last deposit mostly consists of quartz and amethyst crystals which will go towards the free space to form a crystal-lined cavity or geode. Upon the disintegration of the matrix in which the stone are embedded, they will become free. Agates are resistant to weathering and remain in the soil as nodules or they can be seen deposited as gravel in shorelines and streams. This stone appears in nature in several forms. Cyclops agate is a Mexican agate which shows only a single eye. Dendritic agates have fern like patterns which are formed due to the presence of iron and manganese oxides. Limb cast agate is formed by the deposition in a void created by the decomposition of vegetative materials. Turritella agate is formed by the silicified fossil Turritella shells. Agatized coral is also known as Petoskey stone or agate.
Agates are used to attract strength and are believed to protect the wearer from bad dreams. Stress and energy drains can be solved by using this stone. It is good to harden the gums. It is also used for stomach problems. It also cures fever and eye infections. Persian magicians used this to divert storms. Agates are considered to be the lucky stone in love, and bring stability in one's life.
Agates are mainly found in India, Brazil, Madagascar, china, Russia and Australia in colors varying from black to green. Cornelian agate is brownish-orange in color, chrysoprase is green in color, chalcedony is blue, carnelian agate is brownish red and black agate is known as onyx. Being quite brittle, they can crack and break easily even though the hardness is measures 7 in Mohs scale. Most specimens are porous to some degree, and may be dyed to improve its natural color for making ornaments.