Leather Mining Gloves add 1.0 points of mining.
Studded Mining Gloves add 3.0 points of mining.
Ringmail Mining Gloves add 5.0 points of mining.
Leather/Studded gloves are typically considered worthless to the veteran Blacksmith, who will often leave them strewn around NPC shops. Ringmail gloves can often be found for a small fee on player vendors.
The bonus given is not limited by your skill cap, so it is possible to reach a level of up to 105 points with them equipped. Note that you cannot go over 100 skill if you have Jacob's Pickaxe equipped (whether in addition to the gloves or otherwise).
They can be used to aid in skill training. When you are nearly able to mine/smelt a new type of ore, wear the gloves in order to gain the ability early. Remove them when you have gained enough points to smelt the rarer substances without their aid, until such time as you near the next skill threshold point.