Round black agate gemstone
Silver alloy
Drop earrings
2.5 cm length
Agate /ˈæɡət/ is a microcrystalline variety of silica, chiefly chalcedony, characterised by its fineness of grain and brightness of color. Although agates may be found in various kinds of rock, they are classically associated with volcanic rocks and can be common in certain metamorphic rocks."
http://en.wikipedia.org/wiki/Agate
