Key properties
| Chemical formula | Keratin (beta-keratin protein) with embedded pigment granules |
| Crystal system | None (biological, amorphous) |
| Mohs hardness | 2.5 |
| Toughness | Fair; can delaminate at the plate boundaries if dried |
| Refractive index | 1.550 |
| Optic character | Singly refractive (SR, isotropic appearance) |
| Specific gravity | 1.26-1.35 |
| Cleavage | None |
| Fracture | Uneven to splintery; readily sectile (shaves under a blade) |
| Habit | Not applicable; flat overlapping scutes (plates) from the hawksbill turtle's shell |
| Lustre | Dull, resinous; polish: Resinous to waxy, taking a warm soft gloss |
| UV fluorescence | Colourless and yellow portions fluoresce bluish white under LW and SW; dark areas inert |
| Pleochroism | None |
| Cause of colour | Natural pattern of melanin-type pigment clouds in the keratin plate: yellow-amber mottling on rich brown (occasionally black-and-white 'speckled' shells) |
| Typical inclusions | Diagnostic mottled cloudiness: irregular patches of amber-yellow in dark brown; under high magnification, numerous tiny spherical pigment particles; layered plate structure visible at edges |
| Stability | Fair: softens at boiling-water temperature; high heat darkens; burns with a burnt-hair (protein) odour; attacked by nitric acid (unaffected by hydrochloric); may darken with age; dries and checks if kept too dry |
| Heat sensitivity | Extreme for a gem material: it is thermoplastic - softens at about the temperature of boiling water, which is exactly how artisans bent, pressed and welded it |
| Care and cleaning | Warm soapy water and a soft cloth; never ultrasonic or steam; no hot display cases, radiators or car dashboards; hard brushes scratch the soft surface |
| Birthstones | None |
| Availability | No legal new supply: CITES Appendix I since 1975, US import ban, Japan (the last legal buyer) closed in 1992. Only documented pre-ban antiques trade |
| Recognition | Historic luxury material for combs, fans, spectacles, boxes, piqué inlay and guitar picks; now a museum and antique-trade material with celluloid and acetate standing in |
Varieties
Tortoise Shell
| Colour | mottled translucent amber-yellow and dark brown, occasionally black and white |
| Transparency | semitransparent to translucent |
| Clarity type | Translucent plate with internal colour clouds |
| Phenomena | None |
| Appearance | Rich translucent tortoiseshell-brown shot with irregular amber clouds that shift through the depth of the plate; warms and glosses beautifully against the skin |
| Trade names | tortoiseshell / turtle shell • hawksbill shell • 'shell' in the antique trade • piqué/pter work (metal-inlaid shell) |
| Misnomers | 'tortoiseshell' applied to mottled celluloid, acetate, casein or horn imitations (the vast majority of 'tortoiseshell' combs and picks today) • 'genuine shell' for pressed horn |
| Typical sizes | Scutes up to about 20 x 15 cm and about 3 mm thick from large hawksbills; most working pieces smaller |
| Cutting styles | Historically: veneers, inlays, piqué metal inlay, combs, fans, spectacle frames, boxes, guitar picks; heated and pressed into curved forms; joined with a hot iron |
| Key separations | vs celluloid/plastic imitation: the hot point - plastic gives a camphor/chemical smell, real shell burnt hair; plastic shows moulded uniformity or printed mottling ON the surface only, plus flow lines and bubbles, while shell's mottling runs THROUGH the plate; 10x shows shell's spherical pigment specks. vs horn: horn is uniformly fibrous/banded (no amber cloud mottling), SG 1.70-1.85 vs shell about 1.29 - horn is much heavier. vs coated acetate 'tortoise': coating scratches off white |
| Sources (localities) | No legal supply. Historic: Caribbean and West Indies, Brazilian coast, Indian Ocean, Malay Archipelago and East Indies, Pacific - wherever hawksbills nested |
| Availability | Antique/estate only, with paperwork; new hawksbill shell is contraband almost worldwide |
| Market notes | Hawksbills are critically endangered largely because of this trade; CITES Appendix I since 1975, Japanese imports ended 1992, and the US bans import. Sell only documented antiques and use the identification skills to keep buyers honest; modern substitutes (cellulose acetate, Delrin, Tor-tis, Tortoloid) fill the aesthetic niche |
| Care notes | Keep away from heat (softens/distorts at hot-tap temperatures), solvents and nitric acid; stable moderate humidity; store flat; clean with barely damp soft cloth |
| In short | Genuine tortoiseshell - mottled translucent hawksbill keratin - is a banned antique-only material whose depth of mottling, lightness and burnt-hair hot point unmask the plastic imitations that replaced it. |
| Size (ct) | USD per carat | Tier | As of |
|---|---|---|---|
| 0.50–10.00 | $1.00 – $20.00 | documented antique pieces | 2026-10-26 |
Retail market survey, per carat unless noted. Ranges describe what commercial to fine stones trade at; exceptional stones and provenance premiums sit above them.
Sources
Data compiled and fact-checked against gemmological literature, October 2026 (JDMIS gem reference). Properties are ranges accepted in gemmological practice; individual stones vary.
See also
Gemstone reference entries: gem #133
Related materials: Corundum · Diamond · Opal · Quartz · Spinel · Topaz
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