Normal blocks I can mine fine, but when it comes to diamond and gold ores, the block just gets "destroyed" (as in you can't pick it up). I'm playing on multiplayer with no spawning monsters.
A pick made of iron will mine all ores.
A stone pick will only mine coal, and iron; but will destroy gold, diamond, redstone and emerald without dropping the item.
The diamond pickaxe is the only pickaxe to get obsidian.
A wooden pickaxe will only mine coal, diorite, granite, and stone.