Most agates occur as nodule in volcanic rocks or ancient lavas.  Agate is know to fill veins or cracks in volcanic or altered rock.  Such agates, when cut exhibit parallel lines giving it a banded appearance.  You will find many type of agates listed here, all in cabochon form created with the designer in mind.  These and other types of agate will be listed under their called names   Generally an Agate will be transparent but not always, it depends on where the cabochon was cut from the rough.