Organic gems are a different kind of problem from mineral gems. A ruby is a single crystal with a fixed refractive index; a pearl is a layered structure built by an animal, a coral is a skeleton, amber is a polymerised resin. The refractometer is of limited use — many organic materials have no flat surface, and their RI (around 1.5–1.6) overlaps with plastics and glass. Identification rests instead on structure, density, light reactions and, for pearls and some others, radiography.
The organic materials a laboratory sees
Pearl (natural and cultured, saltwater and freshwater), mother-of-pearl and shell, precious coral and its substitutes, amber and copal, jet, bone, horn, ivory (identified but restricted in trade), and — in the Rudraksha lab — seeds and wood, which are organic in the same sense.
The methods
Magnification — the primary tool. Every organic material has a growth structure: the overlapping platelets of nacre; the longitudinal striation and radial cross-section of coral; the flow lines and flattened bubbles of amber; the Haversian canals of bone; the fibrous striation of horn; the Schreger lines of ivory; the granular texture of the Rudraksha surface. Plastics and glass have none of these, and reconstituted materials show grain boundaries and binder. A 10–40× stereo microscope answers most organic-gem questions by itself.
Specific gravity. Organic materials are light: amber and jet near 1.1–1.3, horn 1.3, bone 1.7–2.0, coral and shell 2.6–2.9. Plastic is 1.05–1.4 and overlaps with the lightest of them, but SG separates shell and coral from plastic immediately, and the salt-water float separates amber from most plastics. Specific gravity: weighing a stone against water.
Ultraviolet fluorescence. Amber's blue-white glow, bone's whitish fluorescence, the reactions of resins and adhesives, the uneven glow of dyed material, the chalky fluorescence of some pearl treatments. Supporting evidence in every case.
X-ray radiography. The decisive test for pearls (nucleus type, nacre thickness) and for Rudraksha (internal compartments). Also shows fillers, repairs and internal structure in coral and amber. X-ray of pearls; How X-ray testing of Rudraksha works.
Spectroscopy. Raman spectroscopy separates natural coral pigment from dye and identifies polymer types; FTIR separates amber from copal and identifies Baltic amber; UV-Vis characterises pearl colour origin. These are the instruments of larger laboratories; a bench laboratory notes when a finding would need them.
Drill-hole and surface examination. For every bead: coating layers, dye concentration, core material, wear.
Solvent and hot-point tests. Acetone on copal and dyed coral; a hot needle on plastics and horn. Both are mildly destructive and are used only on hidden spots and only when non-destructive methods have not decided.
Weight, dimensions, photographs. As for every article, so that the report can be matched to it.
The questions the report answers
For pearls: natural or cultured, type of culturing, nacre thickness, treatments. For coral: genuine precious coral or substitute, dyed or not, reconstituted or not. For amber: natural amber, pressed amber, copal or plastic; heat treatment. For shell, bone and horn: the material, and dye or coating. For all: the tests applied and their limits.
What the report does not answer
Age, except in broad terms; precise geographic origin (Basra, Baltic) without spectroscopy; and legality of trade, which is the buyer's responsibility for restricted materials.
Frequently asked questions
Can organic gems be tested without damage?
Yes — magnification, SG, UV and X-ray are non-destructive. Solvent and hot-point tests leave a tiny mark and are used only when unavoidable, on hidden spots, with the owner's consent.
Why is the refractometer not used?
Most organic pieces have no flat facet, and their RI overlaps with imitations. Structure and density are more informative.
Is X-ray used for anything other than pearls?
Rudraksha, primarily; also coral and amber where fillers or internal repairs are suspected.
Can a lab tell antique organic pieces from modern?
It can describe wear, patina and oxidation consistent with age, but there is no dating test for these materials in routine practice.
Which organic material is most often faked?
Pearl, by volume — coated imitations are made in the billions. Amber and coral follow, with plastic and dyed substitutes respectively.