Zircon is a natural zirconium silicate mineral with bright luster, strong fire and double refraction that makes back facets look doubled. It is a different material from cubic zirconia, a synthetic oxide that shares only part of the name. Zircon comes in blue, colorless, yellow, orange, red, brown and green, and blue zircon, mostly produced by heat treatment, is the best-known color in jewelry.
Zircon is not cubic zirconia
The two names cause constant confusion. Zircon is zirconium silicate (ZrSiO4), a mineral that has been mined and worn for thousands of years. Cubic zirconia is zirconium oxide (ZrO2) stabilized in a cubic structure, grown in laboratories since the 1970s as a cheap diamond simulant. Cubic zirconia is singly refractive, while zircon is strongly doubly refractive. Both contain zirconium, which is the only real link between them.
Colorless zircon was used as a diamond substitute long before cubic zirconia existed, which added to the confusion. Natural zircon deserves to be judged as a gem in its own right.
Optical properties
Zircon has a high refractive index, about 1.92-2.01 in well-crystallized stones, and dispersion of about 0.039, close to diamond's 0.044. That gives well-cut zircon a bright luster and noticeable flashes of spectral color, called fire. The page on gemstone optics explains dispersion and brilliance.
Zircon's birefringence is very high, up to about 0.059. Looking through the table of a faceted zircon with a loupe, you often see the back facet edges doubled. This doubling is one of the easiest ways to separate zircon from diamond and from cubic zirconia, neither of which doubles. Cutters often orient blue zircon so the table is perpendicular to the optic axis, which reduces visible doubling through the table. See birefringence and double refraction.
High, medium and low zircon
Many zircon crystals contain small amounts of uranium and thorium. Over geological time, radiation from these elements damages the crystal structure, a process called metamictization. Gemologists divide zircon by how much damage it has suffered:
- High zircon is well crystallized, with the highest refractive index, birefringence and specific gravity (about 4.6-4.8). Most faceted gem zircon is high type.
- Medium zircon is partly damaged, with intermediate properties.
- Low zircon is heavily damaged and nearly amorphous, with lower refractive index (around 1.78-1.85), weak or no birefringence and lower specific gravity (about 3.9-4.1). Low zircon is often green and cloudy and is common in Sri Lanka.
Heating can restore some of the crystal structure in damaged zircon. The radiation levels in gem zircon are low, and stones sold in jewelry are not considered a hazard.
Blue zircon and heat treatment
Most blue zircon starts as reddish brown rough from Cambodia, Vietnam or Thailand. Heating in a reducing atmosphere, with little oxygen, at temperatures near 900-1,000 °C turns it blue. Heating in an oxidizing atmosphere produces colorless or golden yellow stones. This treatment has been practiced in Southeast Asia for a long time, and blue zircon became popular in Western jewelry in the early 20th century.
Some heat-treated zircon can revert slightly toward its original color after long exposure to strong light. Reheating can usually restore it. Heating is standard and generally assumed for blue and colorless zircon; see gemstone treatments.
Sources
The Ratanakiri province of northeastern Cambodia is the classic source of brown zircon that heats to blue. Pailin in western Cambodia also produced zircon, along with ruby and sapphire. Sri Lanka's gem gravels yield zircon in many colors, including low-type green stones. See gems of Sri Lanka.
Other sources include Myanmar, Thailand, Vietnam, Tanzania, Nigeria, Madagascar and Australia. Zircon is hard and chemically resistant, so it survives weathering and collects in river gravels along with sapphire, spinel and other dense gems.
The oldest minerals on Earth
Zircon is a geologist's clock. It takes in uranium when it crystallizes but rejects lead, so any lead inside a zircon formed by radioactive decay. Measuring the ratio of uranium to lead gives the crystal's age. Because zircon resists weathering, heat and chemical change, it can survive even after the rock it formed in has eroded.
Tiny detrital zircon grains from the Jack Hills of Western Australia have been dated to about 4.4 billion years, making them the oldest known minerals of Earth origin. They formed within a few hundred million years of the planet itself, and their chemistry has been used to argue that some liquid water and continental crust existed very early in Earth's history. These are microscopic research grains, not gem crystals. The page on how minerals are classified places zircon among the silicates.
Judging zircon quality
Color is the main value factor. Saturated, pure blue zircon is the most popular, with slightly greenish blue stones selling for less. Red, orange and golden stones have their own following. Most gem zircon is eye-clean. Cut quality matters because good proportions bring out the fire, and the classic "zircon cut" adds extra facets on the pavilion. Large clean stones are available, and zircon offers more size for the money than many other gems. See how colored gemstones are graded.
Durability and care
High zircon ranks about 7 to 7.5 on the Mohs hardness scale, and low zircon is softer. Zircon is brittle, and facet edges can chip or abrade when stones rub against each other. Dealers traditionally wrap zircons separately in parcel papers for this reason, and the resulting wear is sometimes called "paper wear." In jewelry, protective settings help, and rings set with zircon should be removed for hard work.
Clean zircon with warm soapy water. Avoid ultrasonic cleaners and heat, and keep heat-treated stones out of very strong light for long periods.
December birthstone and folklore
Zircon is one of the December birthstones, alongside turquoise and tanzanite. The name is thought to come from the Persian zargun, meaning gold-colored. Medieval Europeans believed zircon helped bring sleep and good fortune, and some traditions credited it with warding off evil spirits.
Frequently asked questions
Is zircon the same as cubic zirconia?
No. Zircon is a natural mineral, zirconium silicate. Cubic zirconia is a synthetic zirconium oxide made as a diamond simulant since the 1970s.
Is zircon radioactive?
Some zircon contains traces of uranium and thorium, but the levels in gem zircon are low, and jewelry-grade stones are not considered a health hazard.
Is blue zircon natural?
The stone is natural, but most blue zircon gets its color from heating brownish rough. Natural blue zircon without heating is very rare.
How can I tell zircon from diamond?
Look through the table with a loupe. Zircon's strong double refraction makes back facet edges look doubled, while diamond shows single, sharp edges.