You will want a relatively low ql pick with as much [[Circle of Cunning]] on it as possible, this will give a lot of skill if you mine [[rock shards]] with it as it heightens the difficulty and the mine time, its the mine time that improves the skill gain in mining, longer mine time = more skill #.
Mining [[rock shards]] is easier for a new player up to a decent skill level (20 -30?) but from then on if you want to grind [[mining]] I would suggest mining on an [[ore]] [[vein]]. At around 30 skill, [[mining]] [[iron vein|iron]] may be the best option for you, but as you get up to around 40 or even 50 in mining, you should mine [[gold vein|Gold]] or [[silver vein|Silver]].
I would not recommend you start off mining on gold or silver because you are very unlikely to get any ore above ql 1.00 (Mining a ql 1.00 ore/shard gives no skillgain whatsoever).