The Forge Lexicon, when placed on a Mystical Plinth, allows you to convert equivalent ores and ingots from one mod's version to another.
After you have crafted the Forge Lexicon, you can shift+rightclick to use or place it on a Mystical Plinth (right click the Plinth with the Forge Lexicon in your hand), a few bolts of lighting will strike the plinth and the book will be removed from your hotbar and placed, floating, above the plinth.
To convert Ingots or Ores place the item in question in the bottom left box. It will then display the name of the item/block in the bottom blue bar and show you the alternatives in the top center box (use the arrow buttons either side, to cycle between the alternatives available for the Ingot/Ore).
Once you have chosen your conversion, click the centre button and the converted Ingot/Ore will instantly appear in the bottom right box (at no cost to the player).