Colombian Emerald

Emerald is the one stone that makes me qualify my own rule. I buy unheated, and with corundum and spinel that ends the discussion. Emerald comes out of the ground fissured, and the trade has been putting oil into those fissures for as long as there has been a trade. Refuse it outright and I would be left with an empty parcel. So the rule bends, to a fixed point and no further: none, insignificant where a laboratory uses that step, or minor. Nothing past that, and no artificial resin at any level.

Two belts and a black shale

Colombian emerald comes from the Eastern Cordillera of the Andes, chiefly in the department of Boyacá, with the eastern belt running on into Cundinamarca. Muzo and Coscuez sit in the western belt, Chivor in the eastern. The names travel further than the stones do. A seller will attach Muzo to anything green and Colombian, and the major laboratories will not attribute a pit — they report a country, and mine-specific provenance is a seller's claim, not a laboratory conclusion.

The geology is the unusual part. Most emerald in the world forms where beryllium-bearing pegmatites meet chromium-bearing schist. In Colombia, hot saline brines moved through carbonaceous black shale and limestone, and beryl crystallised in calcite and pyrite veins, the chromium and vanadium drawn from the sediments themselves. What a buyer sees is iron, or the lack of it. Iron greys a green and drags it towards blue, and there is barely any in Colombian material, so the green reads pure and holds under a warm lamp instead of going flat. Muzo and Coscuez tend to read warm; Chivor runs cooler, towards a slightly bluish green.

Inside, look for jagged cavities holding liquid, a gas bubble and a small cube of halite — three-phase inclusions, long associated with this material. They point towards Colombia without proving it. A gemmologist reads the whole assemblage against trace-element chemistry, and what goes on the report is a country, not a pit.

Why the jardin is not a flaw

Clarity types sort coloured stones by how included the species normally is. Emerald is Type III: material that is essentially always included. The trade gave those inclusions their own name, the jardin. Buyers arriving from diamonds bring the wrong ruler and ask for loupe-clean, which is a category error. An emerald advertised as flawless is either extraordinary or described by someone who has not looked.

Ask instead whether the jardin costs transparency — what the trade calls crystal — or whether the stone still reads open and lit across the face. Then ask whether any fissure reaches the surface, and where. A fissure under a corner is a durability problem before it is a clarity problem. Emerald is hard and brittle at once, which is one reason the step cut named after it has its corners truncated.

The oil scale, and where I stop

Filling works by refractive index. A surface-reaching fissure filled with air behaves like a mirror; filled with something close to beryl, it largely disappears. The traditional substance is cedarwood oil, which sits near enough to work and can be taken out and replaced. It can also dry or seep out over the years. Artificial resins harden inside the fissure instead, which makes them far harder to reverse and, in the short term, usually more stable. Some discolour or craze with age.

Gübelin, SSEF, AGL and GIA all describe the quantity of filler on a scale, but not the same scale. The harmonised terms behind most reports are none, minor, moderate and significant; some houses insert insignificant below minor, and the exact wording differs from lab to lab. The better reports also name the substance itself, and that line is the one most buyers skip. Reading a report properly means separating how much went in from what went in. The lab is disclosing two things, and only one of them is a grade.

Between none and minor there is a real price step, and it widens with weight, because per-carat prices climb as stones get larger and the rarity of an unfilled large emerald compounds with the rarity of the size. Disclosure matters past the invoice, too. A jeweller setting a cedarwood-oiled stone must keep it away from steam, ultrasonic and the torch; set a resin-filled one and the setter cannot see the fissures that dictate how the piece can be handled.

Zambia, the other origin I work

Zambian emerald from the Kafubu area is schist-hosted and iron-rich. That iron shows. The green is cooler, bluish, often more even, and cleaner rough is more common, which more often means less filler — though Zambian stones are oiled as a matter of course too. Zambian material trades below Colombian, which is not the same as being lesser. A well-cut Zambian stone at minor oil is a better possession than a Colombian one carrying moderate, and I sell both, labelled for what they are.

Buy the colour first, the filler second, the country last. When the report says moderate, the stone belongs to someone else.