A clarity-enhanced diamond is a natural diamond whose surface-reaching fractures (often called feathers or breaks) have been masked by the introduction of a high-refractive-index glass into the void, with the result that the fracture becomes optically less visible and the apparent clarity grade improves by one or several steps. The treatment is permanent in the sense that the filling does not naturally migrate out of the stone, but it is impermanent in the sense that heat, certain chemicals, and prolonged ultrasonic cleaning can damage or remove the filler and re-expose the fracture. Disclosure of the treatment is required by major laboratories and by trade nomenclature bodies including CIBJO and the FTC.

The technique

The dominant fracture-filling process was developed by the Israeli diamond cutter Zvi Yehuda in the early 1980s and entered commercial use around 1982. A second process, with a slightly different filler chemistry, was developed by Koss & Schechter. Both processes follow the same general approach: the diamond is cleaned, placed in a vacuum chamber, and infiltrated with a molten lead- or bismuth-bearing silicate glass whose refractive index has been engineered to match closely that of diamond (RI ≈ 2.42). The glass is drawn into surface-reaching fractures by capillary action and pressure, and on cooling it sets within the void. Because the refractive index of the filler approaches that of diamond, light no longer reflects off the fracture surfaces in the same way, and the feather effectively disappears or is greatly diminished.

The treatment does not affect the diamond crystal itself: it does not change the colour grade, the carat weight in any meaningful way, or the underlying clarity of the inclusions that were not surface-reaching. It only addresses surface-reaching fractures and laser-drilled channels (which were themselves a separate enhancement, often combined with fracture filling).

Identification

Major laboratories identify fracture-filled diamonds reliably under magnification. The diagnostic signs include a flash effect, in which the filled fracture shows a flash of orange, yellow, blue, purple, or pink colour when the diamond is rotated under reflected light; a slight cloudiness or trapped-bubble texture within the filled void; a colour change at the fracture site under different lighting; and occasionally a flow structure in the filler. Under fibre-optic illumination the flash effect is particularly clear. GIA, AGS, HRD, and IGI all decline to issue a standard clarity grade on fracture-filled diamonds; instead they describe the stone as "clarity enhanced" or refuse the report and note the fact in their identification reports. EGL and certain other laboratories have at times issued grading reports with a clarity-enhanced caveat, but the major laboratories' practice is well established.

Durability concerns

The principal durability concern is the heat sensitivity of the filler. A jeweller's torch, the heat of casting around a stone, retipping a prong, or even prolonged exposure to a steam cleaner can soften, displace, or in extreme cases expel the filler. Ultrasonic cleaning is generally not recommended for fracture-filled stones, and harsh acids will attack the glass. Reputable retailers selling clarity-enhanced diamonds typically offer a treatment-renewal service, in which the stone is sent back to the treater for re-filling if the original filler is compromised. The lead- or bismuth-content of the filler also raises a small but non-zero concern about long-term wearer exposure, though the quantities involved are very small and the issue is rarely flagged in the trade.

Disclosure

CIBJO's Diamond Blue Book, the FTC's Jewelry Guides, and the trade-association guidelines of the World Federation of Diamond Bourses all require that fracture filling be disclosed at every stage of sale. The required language varies by jurisdiction but typically includes "clarity-enhanced", "fracture-filled", or equivalent terms. Failure to disclose constitutes consumer fraud in most jurisdictions and has been the subject of enforcement actions in the United States, Belgium, and Israel. The treatment is also subject to disclosure requirements at the wholesale level, and reputable bourses require its declaration on dealer-to-dealer invoices.

Place in the trade

Clarity-enhanced diamonds occupy a recognised but specialised niche in the diamond market. They typically sell at 30 to 50% discounts to comparable untreated stones with the post-treatment apparent clarity, and they appeal to buyers who want a larger or apparently cleaner diamond at a lower price and who accept the disclosed treatment and its associated durability constraints. The treatment is more common at the larger sizes (one carat and above) and at the lower clarity grades (I1 to I3 in the post-treatment apparent grade) where the economic benefit is greatest. It is rarely seen on small melee or on stones whose fractures are too narrow or too shallow for capillary infiltration. The category should be distinguished from laser-drilled diamonds (a separate, also-disclosable enhancement) and from HPHT-treated diamonds (a colour treatment, not a clarity treatment).