Hippopotamus ivory is an organic gem material derived from the canine and incisor teeth of the common hippopotamus (Hippopotamus amphibius). Denser and harder than elephant ivory, with a notably finer grain and less conspicuous internal patterning, it has been prized for centuries by carvers working at small scale — netsuke, intaglios, jewellery components, and decorative inlay among them. The material presents as cream to pale ivory-white, occasionally with a faint warm yellow cast, and accepts a high, glassy polish that distinguishes finished pieces from those in elephant or marine mammal ivory. International trade is regulated under the Convention on International Trade in Endangered Species of Wild Fauna and Flora (CITES), and any commercial movement requires documentation.

Physical and Optical Properties

Hippopotamus ivory is composed principally of dentine — the same calcium phosphate mineral matrix (hydroxyapatite) embedded in a collagen protein framework that forms the bulk of all mammalian teeth. Its hardness is approximately 2.5 on the Mohs scale, comparable to other ivories, but its density is measurably higher than that of elephant ivory, typically falling in the range of 1.80–1.97 g/cm³. The refractive index, where measurable on a flat polished surface, is approximately 1.54, consistent with other dentine-based materials.

The most diagnostically useful optical feature of elephant ivory is the Schreger pattern — a cross-hatched lattice of lines visible in cross-section, produced by the interlocking arrangement of dentinal tubules. In hippopotamus ivory, Schreger lines are either absent or so faint and irregular as to be practically invisible, a key distinction in identification. The grain is exceptionally fine and homogeneous, which is precisely the quality that made the material attractive to craftsmen requiring crisp, detailed carving without the risk of splitting along growth planes.

Fresh hippopotamus ivory is white to cream; aged or buried material may acquire a yellow, tan, or brown patina through oxidation and mineral infiltration. The outer enamel cap — present on the tips of canine teeth — is the hardest naturally occurring biological material, reaching approximately 5 on the Mohs scale, and was occasionally exploited in its own right for small hardstone-like applications, though the enamel layer is thin and difficult to work in quantity.

Anatomy and Yield

The hippopotamus possesses four principal teeth of interest to the ivory trade: two lower canines (the largest, curving outward and upward from the jaw) and two lower incisors. The canine teeth are the primary commercial source; in large adult males they may reach 50 cm in length and weigh up to 3 kg each, though usable dentine after removal of enamel and pulp cavity is considerably less. Upper canines are shorter and less commonly used. The cross-section of a hippopotamus canine is roughly triangular, which imposes constraints on the size and geometry of blanks that can be cut from a single tooth — a practical consideration that has historically directed hippopotamus ivory toward small-format work rather than the large plaques and panels possible with elephant tusk.

Historical and Cultural Use

The use of hippopotamus ivory predates recorded history. Archaeological evidence from ancient Egypt documents its use in amulets, figurines, and cosmetic implements as early as the Predynastic period (before 3100 BCE). The famous hippopotamus figurines of Middle Kingdom Egypt — small faience objects modelled on the animal itself — are distinct from ivory carvings, but actual hippopotamus tooth ivory was worked into combs, gaming pieces, and votive objects throughout the Dynastic period.

In Europe, hippopotamus ivory reached craftsmen via North African and Levantine trade networks. Medieval and Renaissance carvers used it for small devotional objects, and it appears in the inventories of several European royal collections. Its particular suitability for fine dental work led to its adoption by eighteenth- and early nineteenth-century dentists as a material for denture bases — a use well documented in the historical record and one that accounts for a proportion of antique pieces encountered today.

In East Asia, hippopotamus ivory was used by Japanese netsuke carvers, who valued its density and resistance to chipping for the fine undercutting that characterises the best netsuke work. Connoisseurs of netsuke distinguish hippopotamus ivory from elephant ivory by its whiter colour, finer texture, and the absence of the characteristic Schreger cross-hatching visible in elephant ivory under magnification.

Identification and Separation from Other Ivories

Distinguishing hippopotamus ivory from elephant ivory and from other organic simulants is a matter of practical importance given differing regulatory regimes. The principal diagnostic criteria are:

• Schreger pattern: Absent or indistinct in hippopotamus ivory; clearly visible in elephant ivory as intersecting arcs, with angles typically below 90° in African elephant and above 90° in Asian elephant.
• Grain and texture: Hippopotamus ivory is finer-grained and more homogeneous; elephant ivory shows a coarser, more layered structure.
• Density: Hippopotamus ivory is measurably denser than elephant ivory; hydrostatic weighing can assist separation.
• Enamel cap: The presence of a hard, glassy enamel layer on the outer surface of a tooth fragment is diagnostic of hippopotamus (or other toothed mammal) origin; elephant tusks are modified incisors covered in cementum, not enamel.
• Fluorescence: Under longwave ultraviolet illumination, hippopotamus ivory typically fluoresces a pale blue-white, similar to elephant ivory, though the response can vary with age and treatment history.

For definitive identification in disputed or legally significant cases, scanning electron microscopy of dentinal tubule morphology, or DNA analysis of residual organic material, can provide species-level identification. Several gemmological laboratories offer such analysis, and the GIA has published reference data on ivory identification methodologies.

Regulatory Status and Trade

The hippopotamus (Hippopotamus amphibius) is listed on CITES Appendix II, meaning that international commercial trade is permitted but requires export permits issued by the country of origin and import permits where required by the destination country. This listing has been in place since 1995, following documented declines in hippopotamus populations across sub-Saharan Africa driven by habitat loss, poaching for meat, and ivory collection.

The regulatory framework is more permissive than that governing elephant ivory — where near-total commercial trade bans apply in most jurisdictions — but it is nonetheless substantive. Dealers and auction houses handling antique pieces containing hippopotamus ivory are advised to obtain species identification documentation where possible. Antique items (generally those demonstrably manufactured before 1 July 1975, the date CITES entered into force) may qualify for exemptions under the antique provisions of CITES and national implementing legislation, but the burden of proof rests with the seller or importer.

In the current market, hippopotamus ivory appears most frequently in the context of antique and ethnographic sales, Japanese netsuke auctions, and specialist natural history collections. It is rarely encountered as a primary material in contemporary jewellery, both because of regulatory complexity and because alternative materials — including fossil mammoth ivory, which is not subject to CITES controls — have displaced it in the trade for carved organic gem materials.

Care and Handling

As a dentine-based organic material, hippopotamus ivory is sensitive to extremes of humidity and temperature, which can cause cracking or delamination along growth planes. Prolonged exposure to direct sunlight accelerates yellowing. Cleaning should be confined to a lightly dampened soft cloth; ultrasonic and steam cleaning are contraindicated. Storage in stable, moderately humid conditions (45–55% relative humidity) is recommended for significant pieces.

Further Reading

• GIA — Identifying Ivory: Methods and Challenges
• Gems & Gemology — Ivory Identification Notes