Blocks
From Hack/Mine Wiki
Block
This mod adds several new block properties. The most noteable is the tower/dungeon blocks. They range in color and vary in use. The first thing you'll notice is that the housing blocks have no breaking animaton. Nothing known can destroy these blocks. However, the inside chambers/levels have differently colored blocks that behave like normal stone bricks.
The obsidan block can be mined by using a golden pickaxe. It can also be created by using the bucket trick. Portals to the nether work well and fully(SMP bugs are few and far between).