I have a Nightstone and a simple grinder, but can't for the life of me figure out how to cut a gem in WoW. What's the right way to do it once you have jewel crafting 300+?
To cut gems, you open your Skills interface and choose the Professions tab.
Click on the Jewelcrafting icon and it will open another interface containing a list of the gems you can cut. Select one and click on the Create button.
However as Jared says, you can't cut Nightstones until you've reached a skill level of 425.
You won't be able to make nightstone gems until you are at least 425 skill in jewel crafting.
Ref: Nightstone gems