Agate is one of the world widest variety of gemstones. Most agates occur as geodes or nodules in volcanic rocks or lava, in former cavities produced by eruptions were filled by siliceous matter deposited in regular layers. Agate is also known to fill veins or cracks in rock. Such agates, when sliced display parallel lines, giving a banded appearance. Such formations are known as banded agate,or striped agate.