Black light test: When you place a real diamond underneath an ultraviolet light, the stone with fluorescence in it will turn blue. But it's important to know that this will only happen with about one-third of all diamonds. A fake diamond, on the other hand, will almost never look blue under a black or UV light.
Only about 25% to 35% of diamonds exhibit some degree of fluorescence in reaction to long-wave UV light. More than 95% of these diamonds fluoresce blue. A rare few fluoresce other colors such as yellow or green.
Natural diamonds have a property known as fluorescence. This phenomenon enables diamonds to produce glows of varied colors when exposed to black light (also known as ultraviolet light). A pure and natural diamond is known to produce a blue glow when exposed to black light.
Diamonds can fluoresce in a variety of colours – blue, yellow, green, orange and white – but blue is the most common.
In most cases, diamonds fluoresce blue. This cancels out the slightly yellowish color in most diamonds, resulting in a whiter appearance in sunlight. In artificial light, there's little UV intensity more than a few inches from the light source.
This kind of staining is the result of radiation damage by alpha particles, which cannot penetrate very deep into the diamond. Radiation stains usually only reach a few micrometers into the diamond and this green “skin” can be lost during cutting and polishing.
You will see that the diamond has white or silvery sparkles only. However, cubic zirconia will give out colourful sparkles such as red, yellow or blue. Another important differentiating feature between these two stones is that a diamond possesses sharp facets, whereas a zirconium does not have sharp facets.
Fluorescence: Sapphires of any color can fluoresce under long wave UV light if iron content is low enough that it doesn't quench fluorescence. Fluorescence colors include pink and red (due to chromium) and orange (due to F-centers or crystal lattice defects according to Vigier & Fritsch, 2023).
The way that diamonds reflect light is unique: the inside of a real diamond should sparkle gray and white while the outside should reflect a rainbow of colors onto other surfaces. A fake diamond, on the other hand, will have rainbow colors that you can see inside the diamond as well.
Fluorescence is the visible light that a diamond emits when it is exposed to the UV rays. Fluorescence is naturally occurring, and at it's best it can make off white diamonds look whiter or more colourless. Around 30% of diamonds evaluated by the GIA have varying degrees of fluorescence detailed on the certificate.
Once your black light is turned on, hold the stone under the UV light. Most diamonds will have a blue-colored glow, but if you see the slight green or yellow fluorescence instead, you'll need to seek the opinion of a professional jeweler as this may mean your stone is not genuine.
The blue nuance occurs in diamonds that are grown through the HPHT (high pressure high temperature) process. Luckily, not all HPHT diamonds have it. During the HPHT process, lab diamonds are sometimes exposed to boron, which speeds up the diamond growth process. Boron impurities are what cause the diamond to turn blue.
Fake diamonds like cubic zirconia do not glow under black light!
An ideal cut diamond shows only a few intensely bright flashes in direct sunlight. A diamond that actually looks good in sunlight splits those few flashes up into ten or twenty smaller sparkles.
Blue light will make some rubies fluoresce. Sunlight gives fine quality Burmese rubies a natural glow from the fluorescence. Don Newsome As Wim pointed out ruby (corundum) does not always fluoresce. And when they do there can be a big difference in the luminance (brightness) of the red fluorescence.
Real pearls will display fluoresce under ultraviolet light, meaning they glow. Press a small UV flashlight over a pearl to see if it glows. If the pearl glows, it's likely a real pearl. An authentic pearl displays fluoresce under ultraviolet light because of the presence of natural organic compounds in its nacre.
Examples of gemstones that can show fluorescence are topaz, opals, rubies, diamonds, citrine, and various colored sapphires. Not every one of these gemstones will fluoresce. Fluorescence is typically a rare occurrence in most kinds of gemstones.
This water test is easy if you have a loose diamond: Get a glass and fill it with water (it doesn't matter what type of water you use). Drop the diamond into the glass of water. Due to the high density of diamonds, when dropped into the water a real diamond will sink.
By holding a diamond close to a source of light, such as a flashlight, you can tell whether or not it's real. Watch closely for the shimmers of light that shine from the stone. A real diamond will reflect light easily, and provide you with a disco ball or rainbow-like display.
Inspect the Diamond
Place the gemstones in a glass of water and watch. If it floats, then it is probably a fake stone. Real diamonds will sink to the bottom of the glass, while cubic zirconia will float to the surface.
Fluorescence is the impact that UV light has on a diamond, caused by the incorporation of natural elements such as boron, nitrogen, or aluminum, during the formation of the diamond billions of years ago.
Red diamonds are the rarest of the colored diamonds, with only 20-30 existing in the entire world. They get their beautiful red color from a rare process during their formation, which changes the crystal structure of the diamond and causes light to pass through it differently than colorless diamonds.
Cloudy diamonds are rare in the upper clarity grades, but not uncommon in the lower clarity grades. Certain types of inclusions and graining can sometimes reduce transparency in a diamond to a noticeable degree. If transparency is degraded enough, the diamond can appear obviously cloudy or milky and lacking in luster.