There are two gain systems for mining, that are really unrelated.
1. Gains from Mining: These are difficulty based, on your chance to successfully DIG ore. At 100 skill, you become 100% able to get ore from any swing, which means that you can't gain from minng anymore. As a result, if you are at 95 real, and wearing the +5 gloves, your gains from swinging a pick/shovel will stop.
2. Gains from smelting: These are difficulty based, but on the chance to successfully SMELT the ore.
Iron Ore starts at 0% at 25 skill, becoming 100% (and no chance to gain skill from smelting) at 75 skill.
Colored ores start at 65 skill for Dull Copper (+5 for each successive metal, except Valorite which is at 99 skill), with a base of 50% for every colored ore, when it becomes accessible. Each of these can be gained on from smelting for 25 skill points after one gets the ability to mine them. So, DC stops giving gains at 90, Shadow at 95, Copper at 100 and Bronze at 105 (smelting with gloves). However, gains are quickest for metals when in the 60-80% range (minimum skill to mine/smelt +5 to +15). Typically, it's better to smelt harder metals for gains, than smelt ones you are 80% or better to smelt.