Vulcanic glass obsidian from collection of National Museum, Prague, Czech Republic (Photo credit: Wikipedia)
Obsidian from Obsidian cliff (Photo credit: Wikipedia)
Obsidian is basically a natural glass, has a fairly soft hardness of 5-6, is semi-easily breakable, and most of all, has that glassy black color that draws you in.
The first stone I ever cabbed was Obsidian, and it has a swirl of purple and green you can only see in sunlight. Most pieces of Obsidian have no color (other than black, of course), whereas some have much more color than others. The ones that have different colors regularly layered inside the stone can be cut so the bands appears to form a heart: The stone is cut in a regular cabochon, so as it slopes down it looks like circled bands around the center which is the highest point. The only exceptions is that a shallow ditch is cut at the top and a small ridge is left a the bottom. This gives the impression that the layers are naturally in a heart form.
One more interesting thing about Obsidian is that when is fractures, it fractures polished, so the rockhounds who mine it know exactly the colors and quality, as well as picture in their mind what the finished cab will look like.