Ask the jeweler to clean the diamond. Cleaning is best done by steaming or by ultrasonic cleaner

View the diamond against a white background. Use a diamond grading trough or a pure white business card to view the diamond against. Tilt the diamond and look at it from the side under a daylight fluorescent light if possible. Look for a yellow tint - the best diamonds are colorless like an ice cube.

Get all facts on the bill of sale. Make sure all pertinent facts - carat weight, color grade, clarity grade, cut, and exact dimensions are put on the bill of sale.

Verify facts with a gemologist. If the diamond is one carat or larger, it should be accompanied by a laboratory report. If not, make the sale contingent on verification of the facts by a qualified gemologist or testing lab.

Decide what is important to you. Most diamond purchases are a compromise. Decide if size (weight), color or cut is more important to you. Know what you are getting and then get what you pay for!