A stone used to sharpen weapons. Talk to Mynoc about his spear (requires decent Perception to notice his is different), then ask Morlis for the flint. She'll ask for three batches of healing powder in exchange. If you don't want to give her the powders, you can talk her out of it (requires Speech). If you are caught stealing it a few times, she'll give it to you. Once you have the flint, you can give it and a spear (you have two at this point) to Mynoc. He'll use up the two to make a sharpened spear.
Morlis has two pieces of flint, but Mynoc will only use one.