The Equivalent Exchange mod is based around alchemy. It adds in a number of extremely powerful end-game items that require large quantities of diamond to make, and large quantities of glowstone to power. It also grants the ability to easily condense materials into more valuable forms, up to diamonds and diamond ore. Most of its recipes are based around the Philosopher's Stone, which you will need in order to access much of the new content.
The EE wiki can be found here.
If you can't find information on a specific Equivalent Exchange feature here, try searching for it in the search box or looking at the Equivalent (see what i did there?) page on the Tekkit Wiki.