Emerald is green beryl coloured by chromium (and sometimes vanadium), the stone of Mercury (Budh) in the Navratna. It is unusual among major gems in two ways: it is almost always included, and it is almost always treated. A clean-looking emerald has, with rare exceptions, been filled. The questions a buyer needs answered are whether the stone is beryl at all, whether it is natural, and how much filler it contains.
What emerald is
Beryl (beryllium aluminium silicate) with chromium and/or vanadium giving a green of sufficient saturation to be called emerald rather than green beryl — a boundary that laboratories draw slightly differently. Properties: RI 1.577–1.583 (two readings, small birefringence), SG 2.67–2.78, hardness 7.5–8 but brittle, doubly refractive, pleochroic (bluish green / yellowish green), chromium spectrum with lines in the red, usually inert or weakly red under UV (iron suppresses fluorescence in most natural emerald).
What is sold as emerald
Green glass. The commonest imitation, sometimes with added inclusions to look "natural". Singly refractive, bubbles, RI variable, no chromium spectrum.
Dyed quartz and dyed quartzite. Crackled quartz with green dye in the fractures; quartzite aggregate dyed green ("Indian emerald" in some old trade usage). RI 1.54–1.55, SG 2.65; dye concentrated in cracks under magnification; dye spectrum.
Green garnet (tsavorite), green tourmaline, peridot, chrome diopside. Each a legitimate stone under its own name, each identifiable by RI and SG; peridot and tourmaline are traditional Upratna substitutes.
Synthetic emerald. Flux-grown (Chatham, Gilson) and hydrothermal (Biron, Tairus, Regency). Same RI and SG as natural, often strong red fluorescence, distinguished by flux veils, platinum platelets, chevron growth, nail-head spicules and seed features. Some very clean synthetics need advanced testing. How synthetic gems are grown.
Assembled stones. Soudé emeralds — two pieces of colourless beryl or quartz with a green layer between — show the join and the colour layer in immersion.
Treatments in genuine emerald
Fracture filling with oil or resin. Nearly universal. Graded none / minor / moderate / significant by extent; oil versus resin identified where possible. Filled emeralds need care in cleaning and setting. Emerald oiling and resin filling.
Coloured filler. Green-tinted oil or resin adds colour as well as clarity and must be stated.
Coating. Rare.
How the laboratory examines a claimed panna
Refractometer and polariscope separate beryl from glass, quartz and garnet at once. Dichroscope confirms pleochroism. Spectroscope confirms chromium (and catches dye). UV notes fluorescence and reveals resin filler. Microscope decides natural versus synthetic and assesses filling — flash effect, trapped bubbles, residue — and grades its extent. Report states species, natural or synthetic, and filling with grade.
Reading an emerald report
Natural beryl (emerald), minor clarity enhancement (oil) describes a desirable stone. Moderate and significant describe stones whose apparent clarity depends on the filler, priced accordingly. Synthetic beryl (emerald), hydrothermal is a different article. Dyed quartz was never emerald. A report that says Natural emerald without addressing filling has omitted the finding that most affects value.
Frequently asked questions
Do all emeralds have inclusions?
Almost all natural emeralds do — the "jardin". A perfectly clean natural emerald is exceptional; a clean-looking one has usually been filled.
Is oiled emerald genuine?
Yes. Oiling is a treatment applied to natural emerald and has been for centuries. It must be disclosed with its extent.
How can I check for glass at home?
Look with a loupe for round gas bubbles, which glass has and beryl does not, and for a lack of the angular, planar inclusions of natural emerald. A laboratory RI reading is conclusive.
Is emerald durable enough for daily wear?
It is hard but brittle, with fissures; corners chip and filler can be disturbed. Rings need care; pendants and earrings are safer.
What is "Colombian" emerald?
Emerald with a stated origin in Colombia, historically the most prized source. Origin is an opinion based on inclusions and chemistry; see Can a laboratory tell where a gemstone came from?.