Equivalent Exchange is a mod for Minecraft that is included in the Tekkit and Technic mod packs. It adds alchemy as a key game mechanic. Currently in version 2 (EE2), a third version (EE3) is in development.
EE2 assigns items an EMC (Energy Matter Currency) value, which increases as the quality of the item gets better. (For instance, blocks of cobblestone are EMC 1, while diamonds are EMC 8192.) Using a Transmutation Tablet, players can turn stacks of lower-cost items into smaller amounts of higher-cost items. Not all mods support EE2, so at present it is still possible to find items which have no EMC value and cannot be crafted using EE2.
EE2 also adds several high-quality endgame items, such as rings with specific powers and high-quality armors and tools, crafted from new forms of matter that can only be created using the Transmutation Tablet.