A cut and polished chalcedony geode, with a scale.
Chalcedony is a transparent to translucent form of quartz. Its name comes from the Greek name of an ancient town in Asia Minor. Chalcedony is found in different colors from white to gray, blue, brown or black. Many of the varieties of chalcedony, including agate, carnelian, bloodstone, jasper, are useful and important as ornamental stones or objects.