Define Agates

History

• Agates get their name from the Sicilian river Achates, now known as Dirillo, where they were mined over 3,000 years ago. Theophrastus, a Greek naturalist and philosopher, gave the stone its name after discovering it along the shoreline of the river Achates between the 4th and 3rd centuries B.C.

Function

• Agate is commonly used as a gem in jewelry making, such as in brooches and pins, and is used to make ornamental objects such as seals, marbles and letter openers.

Types

• There are many types of agate. Fortification agates are banded or striped. Waterline agate and onyx contain parallel bands. Shadow agate displays a shadow effect when rocked back and forth. Tube agates have hollow tubes filled with silica. Eye agates display circle-shaped droplets of crystallized silica. Plume agates contain feather-like inclusions. Geodes have hollow centers. Moss agates have inclusions resembling plants or trees, and seam agates contain cracks within the rock.

Geography

• Most agates occur as cavity fillings or nodules in volcanic rocks worldwide. Fossilized coral can be found in prehistoric ocean beds that have been penetrated by silica-rich ground water.

Size

• Agates range from pea-sized to geodes that can measure a foot across or more. At Lake Superior, in the United States, agate rocks weighing around 20 lbs. have been found, though agates this size are very rare.