Andean Scapes team · Updated
Emerald gemology: what makes a stone an emerald
Green beryl, a few atoms of chromium and vanadium, a garden of inclusions and, often, a little oil. The science behind the green, from the laboratories that study it.
What is an emerald?
Emerald is the green to bluish green variety of beryl, the mineral species that also includes aquamarine.[1]
Where green beryl ends and emerald begins is disputed. Some traders call any green beryl colored by chromium an emerald; most gemologists, laboratories and dealers call a stone green beryl when its color is too light, and GIA uses lab-graded comparison stones to decide.[1]
Where the green comes from
Chromium, vanadium and iron cause emerald’s color; which of them are present, and in what amounts, sets the exact green of each crystal.[2]
The most desirable colors run from bluish green to pure green, with vivid saturation, a tone that is not too dark and color spread evenly through the stone. Too yellowish or too bluish, and the stone is another variety of beryl.[2]
The jardin: why emeralds are rarely clean
Most emeralds contain inclusions visible to the naked eye, often mossy or garden-like. The trade calls them jardin, French for garden.[2]
The trade accepts eye-visible inclusions in fine emeralds; eye-clean stones are rare and especially valuable. Inclusions only cost value when they cloud transparency.[2]
Three-phase inclusions and origin
The classic Colombian inclusion is a jagged pocket of brine holding a gas bubble and a cubic salt crystal, usually 100 micrometers to 1 millimeter long. Jagged inclusions larger than about half a millimeter are unique to Colombia.[3]
Laboratories sort emeralds into two families. Colombian, Afghan and Chinese (Davdar) emeralds form in hydrothermal or metamorphic settings and often have jagged multiphase inclusions; emeralds from Zambia (Kafubu), Russia, Ethiopia and Brazil are schist-hosted and usually have blocky ones.[3]
Oil, resin and the Emerald Report
Surface-reaching fractures in emeralds are often filled with oils, waxes or artificial resins to make them less visible and improve apparent clarity.[4]
The fillers vary in stability and amount, from insignificant to major. A filled emerald should be kept away from heat, sudden changes in air pressure, chemicals and even hot dishwater.[4]
Since 2000 the GIA laboratory has classified emerald clarity enhancement as minor, moderate or significant. The classification describes the treatment; it is not a clarity grade.[2]
Emerald facts
Emerald is beryl, Be₃Al₂Si₆O₁₈, with a Mohs hardness of 7.5 to 8, a specific gravity of 2.72 and a refractive index of 1.577 to 1.583.[5]
Hardness is not toughness. Almost all emeralds have fractures and are more brittle than corundum, so they can be damaged in cutting, setting or careless wear.[2]
Emerald is the birthstone for May and the gem of the 20th and 35th wedding anniversaries.[5]
Frequently asked questions
- Is emerald a type of beryl?
- Yes. Emerald is the green to bluish green variety of beryl, the same mineral species as aquamarine.[1]
- Do all emeralds have inclusions?
- Almost all do. Eye-visible inclusions, called jardin, are normal in emeralds, and eye-clean stones are rare and especially valuable.[2]
- Are emeralds treated?
- Often. Surface-reaching fractures are commonly filled with oils, waxes or artificial resins to improve apparent clarity.[4]
Sources
- GIA Gem Encyclopedia. Emerald Description. Gemological Institute of America.
- GIA Gem Encyclopedia. Emerald Quality Factors. Gemological Institute of America.
- Saeseaw, S., Renfro, N. D., Palke, A. C., Sun, Z. et al. (2019). Geographic Origin Determination of Emerald. Gems & Gemology, 55(4). GIA.
- Weldon, R. An Introduction to Gem Treatments. GIA Gem Encyclopedia.
- GIA Gem Encyclopedia. Emerald (overview and facts). Gemological Institute of America.