Obsidian is a type of block found in the new update 0.28_01. It's color is dark purple and dark blue, it's sort of like the bedrock, but its main color is purple. The texture is possibly rough, from the looks of the block, it looks very rough.
Obsidian (mistakenly known as Opsidian) is never to be found in a map, rumors were that obsidian was a frozen molten rock, which could probably be found near lava. But there's no proof. You can find Obsidian in your Block Inventory (UI) by pressing B. Obsidian is a great block to use for art, especially when you use a lot of black.