Emerald is the one major gemstone for which treatment is the rule rather than the exception. The crystal grows with a network of fissures — the "jardin" that gives emerald its character — and many of these reach the surface. Filling them with a substance of similar refractive index makes them far less visible and the stone far more attractive. This has been done with oils for centuries and with synthetic resins for decades. The question a buyer should ask is not is it treated but how much, and with what.
Why filling works
A surface-reaching fissure is a gap containing air. Air has an RI of 1.0, emerald about 1.58; light hitting the gap reflects and scatters, and the fissure shows as a white or silvery plane. Fill the gap with something whose RI is close to 1.58 and light passes through nearly undisturbed. The fissure does not go away; it becomes hard to see.
The fillers
Cedarwood oil and other natural oils. The traditional filler, RI about 1.51. Colourless, stable enough for years, but it can dry out, leach out in cleaning, or discolour, after which the fissures reappear and the stone can be re-oiled. Widely considered the more acceptable filler because it is reversible.
Synthetic resins (epoxy-type, polymers). Closer RI match, better durability, and — because they harden — much harder to remove. Some are applied with a hardener and become effectively permanent. Resin-filled emeralds are more stable in wear but less honest in one sense: a stone that has been resin-filled cannot be returned to its untreated appearance for assessment.
Coloured fillers. Oil or resin tinted green to enhance colour. This crosses from clarity enhancement to colour enhancement and must be stated separately.
Grading the extent
Laboratories describe the degree of filling on a scale, typically: none, minor (insignificant), moderate, significant (prominent). The grade reflects how much filler is present and how much it affects the appearance — a stone with one small filled fissure is minor; one whose apparent clarity depends on filler in many fissures is significant. Price follows the grade closely, and a stone described as minor that a laboratory grades significant has been mis-sold.
The grading is a judgement, and different laboratories can differ by a step. A report should state the scale used.
How the laboratory detects filling
Flash effect. Filled fissures viewed at an angle show a flash of colour — orange or yellow for oil, blue and orange for many resins — from the small RI difference at the boundary.
Trapped bubbles and flow structure. Filler that did not completely fill a fissure leaves bubbles; some resins show flow patterns.
Whitish residue. Dried oil at the mouth of a fissure looks chalky.
Fluorescence. Many resins fluoresce blue-white or chalky under long-wave UV, while cedarwood oil is weaker or different. UV helps distinguish oil from resin and shows the distribution of filler through the stone.
Hot-point and solvent tests are avoided because they can damage the stone or remove the filler.
Where the distinction between oil and resin matters, advanced laboratories use infrared spectroscopy to identify the filler chemically.
Care implications
Oiled emeralds should not be cleaned ultrasonically or steamed, and should be kept from solvents, heat and detergents, all of which can remove the oil. Resin-filled stones are more robust but should still avoid ultrasonic cleaning and jeweller's heat. A report's care note is worth following.
What a report should say
Species (natural beryl, emerald), the presence of filling, its extent on a stated scale, and — where determined — the type of filler. Natural emerald, moderate clarity enhancement (resin) is a complete statement. Natural emerald alone is not.
Frequently asked questions
Are untreated emeralds available?
Yes, but they are uncommon and expensive, and most have visible fissures. An untreated emerald that looks clean is rare.
Is oiling a defect?
It is a standard, disclosed treatment that most of the trade considers normal. It becomes a problem only when undisclosed or understated.
Can the treatment be redone?
Oil can be replaced by a competent cutter or laboratory; the process is routine. Resin usually cannot be removed and is not re-done.
Does filling affect durability?
Filling does not strengthen the stone. The fissures remain, and emerald is brittle at the corners regardless. Handle with the same care as any emerald.
Why does the extent grade vary between labs?
Because it is an assessment of how much filler affects appearance, and there is no instrument that outputs a grade. Reputable laboratories converge in most cases and differ by one step in borderline ones.