I tend not to alchemize too much in the begining, probably because if I start with fire or earth income, I've plans for it.
Fire I'll alchemize before earth - both because the gold income is better, and because, as Norfleet says, earth just seems more useful.
Usually I'll alchemize 1-5 gems at a time, usually just so I can either get an important unit (typically one of the uber-commanders - Anathemant Dragons, Niefel Jarls, Vanadrotts, etc), or to start construction on a key fort, temple, or lab.
But I also tend to stockpile my gems, if I don't have an immediate use. Meaning that in the mid to late game, I may alchemize 100 out of 300 fire gems, and 50 out of 350 earth, to push my recruitment, building, etc, at a moment when I really need it.
(Really wish there was a keystroke equivalent for selling gems at such times - pressing the up arrow would be -so- much nicer than 150 mouse clicks.)
I half suspect it's a flaw in my play, that I sometimes build up too many gems - I know I didn't use to forge enough, I may not summon enough, and I'm not sure on the subject of empowerment.
