The best tool for harvesting Crystal is a Pick. The Pickaxe and Hatchet will do an okay job, but if you want to get the best bang for your buck, it's best to use a pick. I tested this out and received 13 crystals on average when harvesting with a steel pick. However, whilst using a steel hatchet, I only got 6 crystals, which is significantly less. Using the right tool makes sure you make the best use of your time, so I always recommend using a pick to harvest Crystal.   A player harvesting Crystal on The Isle of Siptah