What Gemstones Show Fluorescence Under UV Light?
What would happen if you put rocks or gemstones under a blacklight? Would they fluoresce? If you've been in a room lit by a blacklight, you might recall seeing things like paper, plastic, and teeth appear to glow. Banana spots, olive oil, certain flowers, and scorpions all fluoresce under UV light as well. What gemstones show fluorescence under UV light?
Only 15% of all mineral species fluoresce, and not every specimen that can fluoresce does so. This means that only some gemstones that contain these minerals show fluorescence under UV light. Different minerals can make them fluoresce in a stunning array of colors. Some emeralds can fluoresce purple! Let's take a look at which gemstones fluoresce and what it means.
Glowing Gems in Legend and History
Throughout history, people have told stories about various kinds of glowing gems. While many of these stories were exaggerations to engage audiences, there's sometimes a scientific link to them. Some of the glowing gems in legend do have fluorescent properties.
India - Vishnu's Light
India was the earliest country to recognize fine gemstones, which hold symbolic value in Hindu texts. The Vishnu Purana states that Vishnu, in his avatar form as Shesha, the multi-headed snake, has a jewel on each head that gives off light.
The Hindu classic Mahabharata describes the raja Babruvahana's palace as dazzling; it contains precious stones that "shine like lamps."
Rubies, the most revered gem in ancient India, have both luminescence and fluorescence. Under UV light, they can appear to glow radiantly. Another important Hindu gem, Diamonds, can phosphoresce, and there is evidence the Indian king Bhoja (r. 1010–1055) knew about this.
Greece and Rome - Gems That Brighten the Night
The Greek historian Herodotus wrote that The temple of Heracles at Tyre had a column made of green gems that shined brightly at night. (Geologist Stanley Hobart Ball guesses that the priests there probably put a candle inside the glowing column to mislead the gullible.)
The Roman natural philosopher, Pliny the Elder, describes a gem called the chrysolampis, or 'golden gleam,' as a pale gold that turns fiery by night.
Pliny also writes about the gem lychnis. An ancient statue of the Syrian goddess Atargatis had the gem on her forehead, which is said to have shone at night like a bright fire. According to Pliny, the gem is called lychnis (Greek for lamp; light) because the light of a lamp enhances the gem's glow, revealing its beautiful hues.
It could be that some of these stories were inspired by fluorite. The term "fluorescence" derives from "fluorite," specimens of which can be highly fluorescent. Many fluorite specimens have a strong enough fluorescence that you could hold them in the sunlight, then move them into the shade and notice a color change.
Egypt – Luminescent Mines
The forbidden island of Ophiodes (also called Topazios) contained King Ptolemy II Philadelphus's famous mine. Miners extracted what they called "Topazion", which were beautiful green gemstones. According to Greek historians Strabo and Diodorus Siculus, the snakes on the island made mining dangerous.
The gems blended in with their surroundings in the day, but at night, the miners could see its illuminating sparkle. They would mark the locations of the gems, and then extract them the following day.
The "Topzaion" gems that the miners extracted were actually peridot. Mines on this island (now called Zabargad Island) still contain many igneous rocks and gems, including peridot. Olivine is not luminescent, although true Topaz is.
It's likely that Egyptian merchants told astounding stories like this to enhance the value of their gems. This helped peridot gain recognition since people hadn't seen it before. Up until the 18th century, people confused topaz and peridot.
If only the Egyptian miners could set foot in the Sterling Hill Mine today, which actually contains fluorescent rocks!
The Fluorescent Mine
In New Jersey, a 2,670 feet deep mine contains the world's largest collection of fluorescent rocks. Visitors follow a rainbow tunnel down into the mine, the walls of which glow in vivid reds, greens, blues, and yellows. At the end of the tunnel, they enter the museum, which contains the world's largest collection of fluorescent rocks.
A lighting sequence shows you how the different rocks look under a blacklight, then a long-wave ultraviolet light, and under both. After, the light turns off, allowing you to see the "afterglow," or phosphorescence, of some of the rocks.
In the legend, Egyptian miners used the glow from gems to find and extract them. Geologists who work at the museum actually use a similar mining technique! To find rocks to add to the collection, they use UV light to find rocks that glow under the light and then extract them.
What Causes Glowing Gemstones?
If you've ever seen a gem appear to glow, you've witnessed gemstone luminescence. This is a phenomenon that occurs when electrons in certain atoms of a crystallized mineral absorb energy and release it. If the energy source is light, it's called photoluminescence, and if the energy source is heated, it's called thermoluminescence.
Ruby and topaz are two of the most photoluminescent gemstones. They appear to glow subtly in indoor or outdoor lighting. Some of the glow you're seeing is actually photons emitted from electrons in the gem – so yes, they do "glow!"
Calcite is an example of a thermoluminescent mineral. It's white, but you put it on a hot plate, it glows blue. Spodumene is another example of a thermoluminescent mineral.
Luminescence is probably the phenomenon that people in the past witnessed. Without understanding the cause, it may have seemed magical. It's no wonder luminescent gems inspired tales and legends of glowing gems.
Gemstone Fluorescence is a form of gemstone luminescence. It occurs when a source of radiation (i.e. UV light) excites electrons in the gemstone, causing them to jump to a higher energy level. The electron stays in this state for a short period of time and then returns to its ground state.
As it returns to its ground state, the electron releases energy, either as heat or as light. Fluorescence is the emission of light. Unlike some other forms of luminescence, fluorescence causes an immediate release of energy.
Rubies are particularly stunning under UV lights. They're one of the few gemstones that have both daylight luminescence and fluorescence.
Some rubies even exhibit visible fluoresce in daylight, from the sun's UV rays. Fluorescence can make a ruby's color even more stunning, increasing its value.
When a gemstone or mineral continues to glow after you take away the light source, this is called phosphorescence. The electrons in the mineral stored the energy from the UV light and then re-emit the light on a delayed basis. This delay is due to the electrons returning to their ground state more slowly.
If you ever had glow-in-the-dark toys, stickers, or clothes, you witnessed phosphorescence.
An example of a phosphorescent gemstone is the blue Hope Diamond. The Hope Diamond phosphoresces a strong red color for several seconds after exposure to a short-wave UV light. Other type IIb diamonds, which only make up 0.1% of all natural diamonds, also phosphoresce.
Fun fact: Rose's "The Heart of the Ocean" necklace in The Titanic is based on the Hope Diamond.
Gemstone triboluminescence refers to the phenomenon where gems emit light when they are struck, scratched, crushed, or rubbed. It's not fully understood yet, but it's most likely that mechanical energy causes the separation and unification of static electrical charges.
Robert Boyle (1627-1691) wrote about his experiments at night with a diamond. He discovered that some diamonds, when rubbed against hard substances (or even against soft ones like cloth) produce a spark.
Other accounts of diamonds producing light in the dark exist before the official discoveries of triboluminescence. It makes you wonder how many people in ancient times ever saw their gemstone produce a spark at night, and then believed it to hold some mysterious power.
What Gemstones Show Fluorescence Under UV Light?
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.
What Causes Fluorescence in Gemstones?
Only 15% of minerals fluoresce, and in order for a gemstone to fluoresce, it must contain some of these minerals. Usually, these minerals must have impurities, known as "activators," in order to fluoresce. Some fluorescent minerals fluoresce reliably, while others are unpredictable.
Examples of activator elements are Chromium, Titanium, Yttrium, manganese, and rare earth elements (REE). The types of metal ions that cause gems to fluoresce are often the same types of metals linked to magnetic susceptibility.
In addition to activators, there are "quenchers." These are impurities that prevent fluorescence if they're present above trace amounts. Mixtures with iron or Nickel, for example, can prevent fluorescent minerals from glowing. On the flip side, if there are too many activators, they can prevent minerals from fluorescing.
It's due to varying concentrations of activator and quencher elements that even fluorescent gemstones don't all fluoresce the same. A ruby with a higher concentration of Iron, for example, will have a weaker fluorescence than one with less Iron.
Testing Gemstones with UV Light
When it comes to identification and evaluation, what does fluorescence mean in gemstones? Testing gemstones with UV light can help you understand more about the gem.
Testing gemstones with UV light can help you determine what it is, if it's natural or lab-grown, if it contains synthetic filler, if it's been heat-treated, or what region it came from. Because there's so much variability, these tests alone are never definitive when it comes to identifying gems.
Some of the most valuable natural rubies fluoresce brightly, but others do not, due to their Iron levels. Synthetic rubies aren't worth as much as natural ones, but they generally have strong fluorescence. This is something that makes it difficult to determine test results for red and pink gemstones.
While testing for fluorescence doesn't work well for green gemstones, it can help you determine if a green gem such as an emerald contains fillers. This is because the filler will often fluoresce whereas the emerald would not.
A filler is an artificial resin used to cover up inclusions. Since emeralds are more prone to inclusions, it's more likely that someone would get them filled with a resin to make them look more perfect. Sellers are obligated to disclose any use of fillers.
Fillers typically fluoresce a whitish color. A good indication that a gemstone contains a filler is if there's a white fluorescent patch in a particular part of the gemstone rather than evenly throughout.
Natural emeralds can fluoresce, but they would have a red to inert reaction under long-wave UV, and a weaker inert or green reaction under short-wave UV. In either case, it's a different color than the fluorescence of the filler. Synthetic emeralds may also fluoresce red, but they wouldn't have the same change under the two types of UV light.
An Involved Process
Testing gemstones under UV light can be very helpful, but the degree of helpfulness depends on the type of gemstone and what you wish to determine.
There are detailed charts online that describe what each type of gemstone may look like under SWUV lights and LWUV lights. These charts reveal how many different types of colors gemstones may exhibit fluorescence. Even if some gemstones can possess strong fluorescence, there may be others that don't fluoresce at all.
Is Fluorescence Good or Bad in Jewelry?
In many cases, fluorescence in gemstones can heighten the beauty of their color and add to the gemstone meanings. Rubies and sapphires are good examples.
In gems such as diamonds, fluorescence can either improve or reduce the quality. A light blue haze, for example, can improve the diamond's clarity. Too much fluorescence can make the diamond appear hazy. Whether or not fluorescence can improve a gemstone or not depends on the types of minerals, activators, and their amounts.
Knowing what gemstones show fluorescence under UV light may or may not influence what kind of gemstone jewelry you buy.
Whatever you decide, fluorescent gemstones will always be a subject of intrigue. There are many specific occurrences that must occur in the earth to create this phenomenon. Fluorescent gemstones are a wonder of nature.
For more information on gemstones, including facts and comparisons, check out our blog.