Diamond and obsidian are easy to find while mining. The best way to find diamond is to branch mine at layer 12. A good branch mine has branches in each direction, with two blocks between branches. Obsidian is created when water is poured onto lava, and can only be mined using a diamond pick axe. Obsidian is easy to find in caves, simply by pouring water into lava pools. A slightly more advanced method of getting obsidian is the "Obsidian Generator." This utilizes a bug to create obsidian from Redstone.