Inkjet and marker coating refers to the application of fugitive dyes, inks or pigmented coatings to the surface of a diamond to alter its apparent colour, principally to make a yellow or brown diamond appear more white or to introduce a fancy-colour appearance to a near-colourless stone. The treatment is one of the oldest and most basic forms of diamond colour modification, predating modern coating technologies (such as the chemical vapour deposition coatings discussed in related entries on diamond treatments) by centuries, and it remains in occasional use particularly in the lower end of the market and in some forms of fraud against buyers who do not detect the surface application.

The basic technique

The simplest form of inkjet/marker treatment is the application of a blue ink or pen mark to the pavilion or culet of a yellow diamond. The complementary blue colouration partially neutralises the yellow body colour through subtractive optical mixing, making the stone appear visually whiter when viewed face-up. Marker pens and India ink have both been used for this purpose, and the technique is documented in dealer-protection literature going back to the early twentieth century.

More sophisticated forms of inkjet/marker coating use pigmented or dyed lacquers, applied to the pavilion surface in patterns designed to enhance the optical effect. The treatments are typically applied after final polishing of the stone and are generally not visible without close inspection, particularly when the stone is mounted in a setting that obscures the pavilion.

Detection

Inkjet and marker coating is among the easier diamond treatments to detect, because the coating is a surface application rather than a structural modification of the stone. Standard detection methods include:

• Cleaning with solvent: Most ink, marker and dye treatments can be removed by cleaning the stone with alcohol, acetone or other solvents. A diamond whose colour grade improves substantially after cleaning has been ink-coated.
• Magnification: Examination of the pavilion surface under standard gemmological microscope magnification (10x to 30x) usually reveals coating deposits, particularly at facet junctions and on the culet where ink can pool.
• Ultraviolet examination: Some inks and dyes fluoresce under UV light in patterns that reveal the coating.
• Standard cleaning before grading: Major laboratories including GIA, AGS, IGI and others routinely clean diamonds in ultrasonic and chemical-cleaning steps before grading, which removes most surface coating treatments. The published grade therefore reflects the cleaned stone's true body colour.

Because of the ease of detection, inkjet/marker coating is not a viable strategy for fraud against any laboratory-graded diamond, and the principal context in which the treatment is encountered today is in trade between dealers or in retail sales where laboratory grading has not been performed. Buyers acquiring diamonds without GIA or equivalent reports should specifically check for surface coating as part of basic due diligence.

Disclosure requirements

Like all diamond treatments, inkjet and marker coating must be disclosed under FTC Jewelry Guides, CIBJO Diamond Blue Book and analogous frameworks worldwide. Sale of an ink-coated diamond as natural-colour without disclosure constitutes a consumer-protection violation and, in many jurisdictions, criminal fraud. The detection methods are sufficiently straightforward that most established dealers and laboratories identify and disclose any surface coating they encounter, and intentional non-disclosure is generally restricted to fraudulent operations rather than mainstream trade practice.

Distinction from CVD coatings

Inkjet/marker coating should not be confused with the more sophisticated chemical vapour deposition (CVD) and physical vapour deposition (PVD) coatings that have developed in the diamond trade since approximately 2000. CVD diamond colour-improvement coatings, including those producing pink, blue and other fancy-colour appearances, deposit thin films of optically active material on the diamond surface using high-temperature vapour-phase processes. These coatings are more durable than ink/marker treatments and are more difficult to detect and remove, although modern laboratory equipment including specialised UV-Visible spectroscopy and detailed microscope examination identifies them reliably.

The boundary between primitive ink/marker treatments and modern CVD coatings is technological rather than conceptual; both are surface treatments that alter apparent colour without modifying the underlying diamond. The disclosure requirements and the trade implications are similar, although the detection methods differ.

Trade implications

For the working trade, inkjet/marker coating is principally a due-diligence concern at the lower end of the market and in trades involving non-graded stones. Standard cleaning, microscope examination and (where doubt exists) referral to a laboratory provide reliable detection. The treatment does not affect diamonds in the laboratory-graded mainstream of the trade, where cleaning protocols routinely remove any such coatings before grading.