I did gems from exactly the value they turned orange on, and gained 5 points (or continued until my ingredients ran out), then started on a new gem. I belive this might be more expensive counting the material loss, but I also belive this way is much faster.
from what i have heard the cheapest way is to craft on orange stuff(about 10points above your skill)
and the fastest is to use yellow ones (or even blue from what i have understod) but using these will result in allot more sucsessfull atempts w/o ganing skill and just get sold back to merchants but still faster as they are low con.
i myself started on gems 10+ my skill and crafted them untill they went the same lvl as myself. then picked a new one.
For me the yellow stones went quite fast but some also have experienced that the orange cons are ok. I just had too many failures on oranges so I sticked to the yellows. Normal crafting is indeed +10 from your skill and craft it till it's -10/15.