A crystal is the clear cover over the dial. Sapphire and mineral are the two covers people compare. Sapphire is harder, and a mark on it tends to be a fine line. Mineral marks more easily, and the mark looks cloudy.
After a swap, look at the lip where the crystal meets the bezel and whether the crystal sits flat. A crystal that rocks under a fingertip is a fit problem. It does not identify the name on the dial.