Agate definition: Agate is one of the primary categories of stones.  It is one of the most popular of crystal beads, especially blue agate, blue lace agate, Botswana agate, brown agate, crazy lace agate, Lake Superior agate and moss agate.  Light can pass through an agate unlike a jasper, another primary category of crystals.  Agates are layers of chalcedony or some other cryptocrystalline silica substance and quartz.  The alternating layers give it a banded appearance.  

It is considered fine grain in appearance in that you can't see separate particles that constitute the layers.  This is called microgranular.  Agates are pretty hard so they are often used for carving.  

Agates form inside bubbles, holes or other types of cavities.  They are usually created by lava activity, but not always.  They do need a cavity though, such as a nodule in volcanic rock or in super-old lava.  There are typically nodules in the rocks that melted from the lava flow.  As water containing silica goes in and out of the cavities in those rocks, the silica gets deposited in layers like layers of skin covering the interior surface of the cavity.  Sometimes this happens where there are cracks instead of cavities, which is why there can be agate veins.  Or layers can build up in the spaces between rocks masses that shift.  The water with the silica or quartz in it was probably the vapor that got trapped as the lava flow cooled.

When you slice an agate stone open, you see the bands or parallel lines.  These are banded agates or striped agates. 

Theophrastus found what we call an agate stone on the band of the Achates River in Sicily.  Theophrastus was a Greek philosopher and naturalist.  This was around the 3rd or 4th century BC.  He was the one to coin the name agate.