Not to be confused with the Rock Cutter, a tool added by GregTech that is essentially an unbreakable Silk Touch pickaxe fueled by electricity.
The Rock Crusher is 2x3x2 machine added by Railcraft, used to process various materials.
The Crusher is built out of 12 special blocks made from a Steel Block, Pistons, and Diamonds. If broken down, it takes 3 Blocks of Steel, 12 Pistons, and 12 Diamonds to build one Rock Crusher. Rock Crushers cannot be built adjacent to each other -- there must be at least one tile in between Rock Crushers.
The Crusher can be loaded and unloaded via Loaders, Buildcraft pipes, or Applied Energistics Export & Import Buses. The entire 2x3x2 structure is treated as a single block, similar to a large chest. It will accept input from the top and will output from any remaining side.
If you have Buildcraft 3.x installed, the Rock Crusher will need to be powered via Buildcraft Pneumatic Power. It will draw 15 MJ of power while processing materials. It can also take RF after its been updated.
If you have IndustrialCraft2 installed, the crusher will also be able to crush ores into dust just like a Macerator.
The list of materials that can be processed includes just about any vanilla blocks made from stone or brick. Additionally, you can process Bones and Blaze Rods, also many of the materials have a small chance of creating bonus material but one of the most important products it can produce is Obsidian Dust. Obsidian Dust has a 25% chance of being produced when processing Obsidian in addition to producing Crushed Obsidian. Crushed Obsidian can be further processed directly into Obsidian Dust if you desire, but it also makes an excellent ballast material.