[Edi]

There is no in-game compendium in the style of Civilopedia. You are limited to what your mages can do with their current levels. Gem cost is directly related to levels, being 5, 10, 15, 25, 40, 60 (i.e. Fibonacci series, each level costs as much as the two previous ones put together).

If you want a complete list of items and levels, take a look at the Dom3 DB spreadsheet file. See the FAQ for details about waht it is. Relevant links are also in my sig. There's a page with all of the items and their level requirements, gem costs and other properties there.