Carat weight: 1 carat = 200 milligrams = 6.5 mm diameter. Doubling weight doesn't double diameter.

Diamond clarity: FL/IF/VVS/VS = super expensive, near perfect. SI = best value if you can check a photo for obvious inclusions (defects).

Color: D-G = colorless, expensive, only if you have money to burn. H-J = best value. Can go lower in gold metal settings than white metal.

Cut: Better cut ratings let more light into a diamond, making it sparkle more. Very important property.

Advice: Set a budget, and a minimum cut (at least Premium). Go J color for gold and I or H for white metals. Go searching for SI1/SI2 clarity diamonds at James Allen, and pick the diamond with no or small inclusions. Pick a ring setting and buy it risk-free (60-day return policy).