This is actually semi precious gemstone Brazilian Agate that has been dyed a rich black! The natural white lines come through and make a beautiful specialty designer cabochon known as Tuxedo Agate.
Sometimes the dye does not get black and is more brownish. We like to call those cabochons Tortoiseshell Agate.
Agate is a form of Chalcedony, or a microcrystalline quartz. It forms in concentric layers in a wide variety of colors and textures. Each individual agate forms by filling a cavity in host rock.
As a result, agate often is found as a round nodule, with concentric bands like the rings of a tree trunk. The bands sometimes look like eyes, sometimes fanciful scallops, or even a landscape with dendrite trees.
In many traditions agate is believed to cure the stings of scorpions and the bites of snakes, soothe the mind, prevent contagion, quiet thunder and lightning, secure the favor of the powerful, and bring victory over enemies.