How to Test Diamonds With a Black Light

Many real diamonds will glow blue under a black light.

Diamonds have long been prized for their beauty, investment value and rarity. Since real diamonds are expensive and hard to come by, there are many fakes on the market. These fakes can look just like real diamonds but are far less valuable. Many of them are synthetic. You can test your diamonds under ultraviolet light (also known as black light) to help determine whether they're real or synthetic.

Things You'll Need

• Ultraviolet light
• Another diamond or known fake for comparison, if available

Instructions

1. Hold your diamond in one hand or place it in front of you. Turn on the black light and hold or place it near the diamond so the diamond is within the glow of the black light.

2. Darken the room by closing the curtains and turning off the lights. If the room isn't dark enough, take the diamond and black light into a closet or small, windowless room and close the door.

3. View the diamond closely. It will most likely have a faint blue glow if it's a real diamond; however, the better the quality, the less blue light will be visible. Fake diamonds or cubic zirconia will usually not glow blue.

4. Compare the glow from your diamond to another diamond, if you have one, that you know is real. You could also compare the glow of the diamond to a known fake or cubic zirconia.

Tips & Warnings

• While the black-light test may help you determine whether your diamond is real, the best way to be sure is to take it to a jeweler for examination. The jeweler will be more knowledgeable about ultraviolet testing and will be aware of variations in the colors of a diamond under black light. Diamonds from different regions will have slightly different tints under black light.

• If you have a cubic zirconia of similar size, compare the weight of the two stones. Cubic zirconia weighs far more than a comparable diamond.

• If your diamond is in a setting such as a ring, examine it under a magnifying glass for any stamped marks in the metal. If you see a "CZ," that means it's cubic zirconia.

• Never purchase a diamond unless it is certified by a reliable source. Don't just take someone's word that it it's real. Also, note that the ultraviolet test is not foolproof; all diamonds won't look the same under black light, just as they won't all look the same in daylight.

References

• National Jewelers Supplies:
• Abazias: Tests to Tell Genuine Diamond from Cubic Zirconia