Mohs hardness: 7

Common uses today - Usable and ornamental stone objects such as stone guitar picks, jewelry, ink boats, and because of it's hardness and resistence to acids it's used to make mortar and pestles.

From Wikipedia - Agate
"Agate is a microcrystalline variety of quartz (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 but can be common in certain metamorphic rocks.
Most agates occur as nodules in volcanic rocks or ancient lavas where they represent cavities originally produced by the disengagement of volatiles in the molten mass which were then filled, wholly or partially, by siliceous matter deposited in regular layers upon the walls.

Often agate coexists with layers or masses of opal, jasper or crystalline quartz due to ambient variations during the formation process.
Because of the high polish possible with agate it has been used for centuries for leather burnishing tools."

From Mindat.org
"[Note: Many non-banded forms of chalcedony - such as Moss agate - are often erroneously called 'agates'. True agate is concentrically banded. Mottled and included chalcedonies are more properly called simply 'chalcedony.'
Petrified wood (agatized wood) is the name given to fossil wood where the replacement of the wood is by chalcedony, but the banding in this case is due to the wood structure - not concentric deposition of the chalcedony - and the material is chalcedony, not true agate.]"


Check out the Mindat.org database for detailed information about Agate varieties.