The natural colour of precious coral ranges from white through pink and salmon to deep red, and the deep reds are the scarce and valuable end. Pale coral is plentiful. Dyeing pale coral red is therefore the oldest and most widespread treatment in the trade, and detecting it is the first thing a laboratory does with any red coral. The methods are simple; the point is to apply them.
Where dye collects
Coral is porous, and dye penetrates unevenly. It concentrates in:
- the drill hole, where the interior of the bead is exposed and absorbs more than the polished surface;
- surface pits, cracks and natural cavities;
- the softer, more porous zones of the skeleton, which take more dye than dense zones and show as darker patches;
- the boundary between coral and any filler used to plug holes.
Natural colour, by contrast, is a property of the skeleton and is distributed according to the coral's growth, with the longitudinal striations and the radial cross-section pattern faintly visible through it. It does not pool in cracks.
Methods
Magnification. The primary method. At 10–40×, dye shows as colour concentrated in the features above, sometimes as a distinct rim of deeper colour around pits, and as an overall colour that sits on the structure rather than in it. Natural red coral shows the longitudinal grain running through the colour.
Drill-hole inspection. With a loupe and light, the hole of a dyed bead shows a dark ring of concentrated dye, and often a paler interior where the dye did not reach. A natural red bead is red throughout the hole.
Ultraviolet. Natural coral is usually inert or dull under long-wave UV. Some dyes fluoresce, and dyed coral may glow unevenly, with the dye-rich zones distinct. Not all dyes react; a negative is not a clearance.
Solvent swab. A cotton bud moistened with acetone (or, for some dyes, alcohol) pressed against an inconspicuous spot — inside the drill hole, or on the back of a cabochon — may pick up colour. Many modern dyes are fixed and do not transfer, so a clean swab does not prove natural colour. The laboratory uses this sparingly and only where it cannot mark the piece.
Spectroscopy. Natural coral pigment (carotenoids) has a characteristic absorption; dyes show different features. Raman spectroscopy in particular separates natural from dyed reliably and is used by laboratories with the instrument.
Comparison across a strand. Natural coral strands vary bead to bead; dyed strands are suspiciously uniform.
Related treatments to check at the same time
Bleaching — lightening dark or blotchy coral; may precede dyeing.
Impregnation — wax or resin to fill pores and improve polish; shows under UV and at the surface.
Reconstitution — coral powder in resin; no grain, resin fluorescence. Often also dyed. Red coral (Moonga) covers the full range of substitutes.
What the report says
Natural precious coral, no indications of dye or natural coral, dyed. Where colour origin could not be resolved without advanced testing, the report says so. Reconstituted, bamboo, bone, shell, plastic and glass are named as what they are.
Frequently asked questions
Is dyed coral still coral?
Genuine coral with added colour. It should be sold as dyed.
Can I do the acetone test at home?
You can, on an inconspicuous spot, but a clean swab does not prove the coral is undyed, and some finishes are damaged by solvent. Look at the drill hole under a loupe instead.
Why does natural red coral vary in colour?
Because it grew that way. Variation across a strand, and slight unevenness within a bead following the grain, are signs of natural colour.
Is bamboo coral dyed?
Almost always, when sold as red coral. Its segmented structure and coarse grain identify it.
Does dye fade?
Fixed dyes are stable in normal wear. Some fade with prolonged sunlight or leach with sweat, which is another way dyed coral reveals itself over time.