Regarding: Maker Skill and Dark Crystals

Just my speculation,

Basic gives +2 or -2;
Intermediate gives +2 or +3 or -2 or -3;
Advanced gives +2 or +3 or +5 or -2 or -3 or -5.

All dark crystal increases stats, not decreases. The increment is 2, 3 and 5 respectively, depending on the level on the gem:

http://www.southperry.net/forums/showthread.php?t=1858