... agree with what Fabian said... however, I don't know if there's a guide, but here's a very condensed version.
Generally you'll want both a prefix & suffix on a magic jewel, with some combos being more or less desirable (i.e. Scintillating Jewel of Fervor is more desirable than an Argent Jewel of Carnage, etc.):
Best Prefixes:
Scintillating / Ruby / Ambergris (i.e. 10+ all-resist or ~25+ single-resist fire or lightning)
Crimson / Vermillion (+min/max damage)
Realgar / Ruby (+Enhanced Damage %)
Argent (+Attack Rating)
Best Suffixes:
of Fervor (15% IAS)
of Freedom (-15% requirements)
of Carnage / Bliss (+max/min damage)
of Truth (7% FHR)
There are a few other mods that can be handy too (especially with a good 2nd mod), such as the +strength/dexterity mods, but they're mostly a little bit of a lower tier. I personally find enough specialty-uses for things like Dun/Aureolic (extra mana) or Pearl/Diamond (damage to undead/demons) jewels if they've got an 'of Fervor' suffix that I always hang on to the ones I find. And, of course, lower-rolls on the elemental resists (or poison/cold resists) can be useful at times too with a 2nd mod.
Rares are a little tougher since they have mostly lower-tier mods and are limited to 4 mods (outside of version 1.08), so you'll need 3-4 pretty good mods in combo to really shine (example: Realgar/Crimson Of Bliss/Truth would be pretty epic).
Beyond that, generally you're looking for either a really low level requirement ('of Envy' jewels can make great level 1 twinking gear, 'of Carnage' is great if you're interested in level18 dueling, etc.), or a perfect roll, or both.
Like Fabian said, jewels are essential for crafting recipes, so until you have a feel for which ones you want most, I'd recommend hoarding them all for a while. It won't take you long to have a stockpile of 95% junk with a few good ones thrown in.