As you know by now, prospecting a certain type of ore requires a specific level in jewelcrafting skill. I had previously speculated that the ore became prospectable when you receive recipes for that metal type from the trainer. During my re-skilling process after release I found this was not the case. I took notes on the levels that each ore required as it opened up for prospecting.
* Copper Ore - 20 Skill (when you learn Prospecting from the trainer)
* Tin Ore - 50 skill
* Iron Ore - 125 skill
* Mithril Ore - 175 skill
* Thorium Ore - 250 skill
* Fel Iron Ore - 290 skill
* Adamantite Ore - 325 skill
For those of you contemplating taking up jewelcrafting, the uncommon ores (silver, gold, etc) cannot be prospected. Only the ones on this list are prospectable.