Certificate. For your own peace of mind, ask for a certified moissanite which usually come with each stone. High quality moissanites are usually come with certificate, and the serial number is laser-printed on the moissanite itself. Look for it when you make a visit to your jewelry store.
Get Measured. Make sure you are properly measured to avoid any discomfort. If this is a surprise, try to get your partner's ring size as accurate as possible.
Color. Like diamond, moissanite has color grades. Colorless is the most common color for engagement rings.
Carat. Moissanites are about 7% lighter than diamond. A diamond with 1.00 carat size (diameter 6.5mm) will weigh 1.00 carat, while moissanite with the same diameter (6.5mm) will weigh 0.93 carat. It is common for jeweler to call a 6.5mm moissanite as 1.00 carat moissanite.