I can mine with both the light pick for gem mining (1d4 dmg, 4 lbs), and the gem crafters chisel (1d4 dmg, 1 lb). Whenever I touch a stone it goes through one round of 'mine/too hard/chisel breaks/etc' on the results table, and then stops. I never find a gem during this initial attempt at mining, and happens once per deposit.
After that the next time I touch the deposit, 9 out of 10 times it goes for one round of results and the deposit disappears. The tenth time it acts as if I am mining it twice at the same time (two result messages at once), and then the deposit acts normally, lasting a random amount of time and giving out gems, etc.
Works for greenstone and malachite, gem crafters chisel and light pick for gem mining.