Agate is a translucent variety of microcrystalline quartz. Agate generally forms by the deposition of silica from groundwater in hollow cavities of volcanic rocks. Agate has also been known to fill veins or cracks in volcanic rock. Such agates, when cut transversely, exhibit a succession of parallel line giving a banded appearance to the section. Such stones are known as banded agate, riband agate and striped agate. The gorgeous agates from Brazil are mined from decomposed volcanic ash and basalt in Rio Grande do Sul. These agates can be very colorful when cut & polished. The agate slabs we sell make for great display pieces or gifts. They often are very translucent when back lit.