'''Third rule of mining skillgain:''' Your mining skill level, pick quality and enchantment is pick enchantments are critical to your choice of what ore you mine.
In order to fully optimize your skillgain PER HIT, you need the lowest possible quality pick which will allow you to frequently create ore or rock (in a mine) between 1.01 and 39.99 quality. Each mine-able substance has a different base difficulty. The lowest being rock, then iron, etc. A combination of your skill and your pick's quality will be compared against the dificulty of what you are mining in order to determine the "curve" of skillgain.
HoweverIn other words, a slightly faster it is possible to adjust your skillgain by varying the quality of your pick. A higher quality pick, on a different type vein, iron might end up generating just as much skillgain, as a lower quality pick on rock because you are hitting for skill more often, even if you are getting a bit less skill PER HIT(because the higher quality pick is faster). If you mine on iron at 64 skill with an 80 QL pick, you will very rarely ever get a skillgain tick. With a 28.5 QL skiller pick, you get more frequent skillgains. On lead, at 64 mining with a 28.5 QL pick, you will be constantly failing with "1" results that give no skillgain. With a higher quality pick you will get more skillups.
Another consideration: A quality 30ish pick is going to maintain it's status and it's enchantment FAR better than a QL 10 pick will, and a QL 1 pick will poof if you aren't very careful watching your pick's damage level.