I don't know what the fantastic woman has told you, but here is the deal:
1. Miner can only MINE gemstones, he cannot process them.
2. Stonecutter can only PROCESS the gemstones into gems, but cannot mine them.
If we had made the stonecutter to mine the gemstones as well, the jewelers would gain too much advantage over other professions and they would be millionaires before a month passes...