Obsidian is a vanilla Minecraft block with many uses in Tekkit. Obsidian may also be used as a building material, due to its considerably high blast resistance of 60 (it has been downgraded from 6000 in vanilla Minecraft).
The usual way of making obsidian is cooling off a lava with water by pouring a bucket of water on top of the lava source. In doing this you save 4 Empty Cells (equivalent to 1 Tin) and use one less lava source. To ensure that the obsidian block is not lost, mine the obsidian as water flows onto it. In doing this the water will turn any lava directly underneath into obsidian, keeping recently mined obsidian safe.