Obsidian

Obsidian is a Vanilla minecraft block with many uses in Tekkit, most of which are from the Equivalent Exchange mod. Obsidian may also be used as a building material, due to its considerably high blast resistance of 60 (it has been nerfed from 6000 in vanilla Minecraft).

IndustrialCraft adds a recipe for Obsidian, requiring 2 Lava Cells and 2 Water Cells.

Usage
As of Tekkit 3.0.3, Obsidian can be used to make about 19 items. These are the most common:

Obsidian Pipe

Ender Chest

Gem of Eternal Density

Watch of Flowing Time

Transmutation Tablet

Energy Condenser

Antimatter Relay

Mercurial Eye

These are only a few of the crafting recipies. The others include: World Anchor, Obsidian Stick, REP, Wireless Tracker, Magtube, and the Accelerator.

Note
-Obsidian can be created using EMC or a transmutation tablet Which is cheaper then 4 cells Obsidian's EMC value is 64