Emerald is the green variety of the mineral beryl, colored by chromium, vanadium or both. Fine emeralds show a pure green to slightly bluish green with strong saturation, and nearly all of them contain visible inclusions. Most emeralds are treated with oil or resin to hide surface-reaching fractures, which makes care and disclosure more important than for many other gems.
What emerald is
Beryl is a beryllium aluminum silicate that occurs in many colors. Blue beryl is aquamarine, pink beryl is morganite, and green beryl colored by chromium or vanadium is emerald. Pale green beryl colored only by iron is usually sold as green beryl, not emerald. Labs and markets differ somewhat on where that line falls, especially for vanadium-colored stones from Brazil and elsewhere.
The trace elements that color emerald are rare in the places where beryllium is found. Beryllium tends to concentrate in granite and pegmatite, while chromium and vanadium are found in dark, iron and magnesium-rich rocks and some shales. Emerald forms only where geological processes bring the two together, which is one reason it is scarce.
Physical and optical properties
Emerald crystallizes in the hexagonal system as six-sided prisms with flat ends. Its refractive index is about 1.565-1.602, low compared with corundum, and its birefringence is small. It shows weak to moderate dichroism, bluish green and yellowish green. Under the Chelsea color filter, many chromium-colored emeralds appear red, though the result varies with source.
Emerald rates 7.5 to 8 on the Mohs hardness scale, but its toughness is only fair to poor because of the fractures and inclusions most stones contain. A hard knock can chip a girdle or open a fracture. This is one reason the emerald cut, a rectangular step cut with clipped corners, became associated with the gem: the cut corners reduce stress points, and the shape suits the long prismatic rough.
Jardin and inclusions
French jewelers called emerald's inclusions jardin, meaning garden, because they can look like moss or foliage. Typical inclusions include healed and open fractures, mineral crystals, growth tubes and fluid inclusions. GIA places emerald in its Type III clarity group, the category of gems that are almost always included.
The inclusions are also clues to origin. Colombian emeralds often contain three-phase inclusions: jagged cavities holding a liquid, a gas bubble and a small cube of halite (salt). Zambian stones commonly show mica platelets and tremolite needles. The page on Muzo and Colombian emeralds covers Colombian inclusions more closely.
Where emeralds come from
Colombia
Colombia has produced fine emeralds since before the Spanish arrived, and it remains the source most associated with top color. The main mines lie in the Eastern Cordillera of the Andes, at Muzo, Coscuez and Chivor in Boyaca department. Colombian emeralds formed from hot, salty fluids moving through black shale, a geological setting unlike almost any other emerald deposit. Many Colombian stones have a warm, slightly yellowish to pure green and a soft glow.
Zambia
The Kafubu area of Zambia's Copperbelt became a major source from the 1970s onward. Zambian emeralds tend to be bluish green, contain more iron, and are often somewhat darker than Colombian stones. They frequently have fewer surface-reaching fractures, so many need less oil.
Brazil and other sources
Brazil produces emeralds from several states, including Minas Gerais, Bahia and Goias. The output ranges from commercial material to fine stones, and the 1980s discovery at Nova Era in Minas Gerais added an important source; see gems of Minas Gerais. Other sources include Zimbabwe (Sandawana, known for small, intense stones), Afghanistan's Panjshir Valley, Pakistan, Madagascar, Ethiopia and Russia's Ural Mountains. Ancient Egyptian mines in the Eastern Desert supplied emerald to the Roman world, though that material was generally pale and heavily included.
Treatments
Fracture filling is standard for emerald. Cutters immerse stones in oil, traditionally colorless cedarwood oil, to fill surface-reaching fractures. The oil has a refractive index close to emerald's, so the fractures become less visible. Since the late 20th century, artificial resins such as epoxy-type fillers have also been used. Some resins harden after filling, and some are more stable than oil.
Labs grade the extent of filling, typically on a scale running from none or minor to moderate and heavy, and some identify the type of filler. Under magnification, filled fractures may show a flash of color, trapped bubbles, or a hazy, flow-like texture. Fillers can dry out, discolor or leak over time, and stones can be re-oiled. Green-tinted oils and dyes are not accepted in fine goods and reduce value sharply. The overview of gemstone treatments puts emerald in context with other treated gems.
Trapiche emerald
Trapiche emeralds, found mainly in Colombia's Muzo area, show six arms of dark material radiating from a central hexagon, like spokes in a wheel. The name comes from the Spanish word for the grinding wheels used in sugar mills. The dark arms are carbonaceous material and albite that collected along the boundaries of six growth sectors as the crystal formed. Trapiche emeralds are cut as cabochons or slices to show the pattern and are collected as curiosities.
Synthetics and imitations
Synthetic emeralds have been made by flux growth since the 1930s and later by hydrothermal methods. Carroll Chatham in the United States and Pierre Gilson in France were early commercial producers. Synthetics can be very clean or show their own characteristic inclusions, such as veil-like flux fingerprints or chevron growth patterns. Imitations include green glass, doublets and triplets with a green layer, and green-dyed quartz. Green tourmaline, tsavorite garnet and chrome diopside are natural stones sometimes confused with emerald.
What affects emerald value
- Color comes first. Pure green to slightly bluish green with strong saturation and a medium to medium-dark tone ranks highest.
- Clarity follows. Eye-visible inclusions are normal, but those that threaten durability or break up color lower value.
- Degree of filling affects price. Stones with no filler or minor filling cost much more than heavily filled stones.
- Cut affects brightness and durability. Well-proportioned stones avoid windows and excessive depth.
- Size and origin also matter, with fine Colombian stones commanding a premium when supported by a lab report.
See how colored gemstones are graded for a broader framework.
Caring for emerald
Clean emeralds with lukewarm water, mild soap and a soft brush, then dry them gently. Avoid ultrasonic cleaners, steam, hot water and solvents, which can remove oil or damage resin. Remove emerald rings for hard work or sports, and choose protective settings such as bezels or set stones in pendants and earrings if durability is a concern. A jeweler can re-oil a stone whose fractures have become more visible. The guide to cleaning jewelry safely has more advice.
Emerald in history and folklore
Emerald was mined in Egypt by the Ptolemaic period, and Cleopatra is often linked with the gem in popular accounts. The Spanish brought large quantities of Colombian emeralds to Europe and Asia from the 16th century, where Mughal rulers in India set them in jewels and carved them with inscriptions. Ancient and medieval writers believed emerald could rest tired eyes or reveal truth, beliefs recorded as tradition, not fact. Emerald is the May birthstone and the traditional gift for a 55th anniversary.
Frequently asked questions
Are all emeralds oiled?
Most are. Filling surface-reaching fractures with oil or resin is standard practice. Emeralds with no filler exist but are uncommon and sell at a premium when confirmed by a lab.
Why do emeralds have so many inclusions?
The conditions in which emerald grows, including fluid-rich environments and tectonic stress, tend to trap fluids and crystals and create fractures. GIA classes emerald as a Type III gem, meaning inclusions are expected.
Can I clean emerald in an ultrasonic machine?
No. Ultrasonic vibration and heat can remove oil, damage resin and widen fractures. Warm water, mild soap and a soft brush are safe.
What is the difference between Colombian and Zambian emeralds?
Colombian emeralds tend to be warmer, pure green and often show three-phase inclusions with salt crystals. Zambian emeralds are often bluish green, darker and contain more iron.