How to View Diamonds Under a Blacklight

As the old saying goes, "Diamonds are a girl's best friend," yet knowing you have a quality diamond, not a flawed gem or a fake, can ensure the friendship lasts forever. When paying top dollar for a piece of jewelry, you want to know the diamonds are flawless and real. Black light, or ultraviolet light, testing helps buyers determine the quality of a diamond. Flawed diamonds, diamonds that have been treated with fluorescence and Moissanite all look different from quality diamonds under a black light.

Getting Started

Create a dark environment. In a small room, turn off any lights and cover any windows or other light sources.

Turn on the black light. Begin scrutinizing your diamond.

Look for a blue hue. Diamonds that exhibit a definitive blue color are usually genuine and lack many flaws. Each diamond is a bit different, but if your diamond shows anything from a slight blue color to one that is dark, deep and rich in hue, you have a real diamond.

Look for bright white colors. Flaws in a diamond will appear white under a black light. If your entire gem is bright white it is possible that it is not as pure as you would have hoped. Stones subjected to a lot of fluorescence also appear bright white. You can check the fluorescence rating on the diamond's certificate; it will be rated on an EGL USA scale. Just because a diamond appears white under a blacklight, that doesn't mean it is fake or flawed. The blacklight test just gives an indication that it could be fake or flawed.

Avoid stones that are green, yellow or gray. If the diamond is even slightly yellow, gray or green under a black light, the stone could be Moissanite. Moissanite is created by humans from silicon carbide and is not a true diamond.

Turn on the lights. If your diamond showed up white while under the black light, look for a cloudy or hazy appearance in your gem. If you do see any haziness, this is an indication your stone is of a lesser quality.


While a stone that shines white under the black light may not always mean that it has been heavily dealt fluorescence, or that it is not real, it may be a sign. There are other methods to check, such as X-ray tests, heat tests and weighing.


Use a diamond dealer that you trust and that is reputable. A black light test is only one of many things to consider using when you are buying. You should look for a diamond certificate and other forms of rating your gem as well.