Ivory

Ivory is the dentine material of tusks and certain other large teeth. It is a mineralized biological tissue, rather than the enamel coating of a tooth or an undifferentiated piece of bone. In elephant ivory, mineralized collagen fibres form a spatially organized composite: identifying its chemical ingredients does not specify their arrangement.^[ref]

A 2018 structural study combined X-ray scattering, observations with polarized light and a scanning electron microscope. The measurements found changing fibre orientations from the outside toward the inside of the tusk. Mid-dentine fibres lay mainly along its length in layers with oscillating orientations. The authors related this arrangement and collagen birefringence to the visible Schreger pattern, rather than treating light and dark regions as necessarily different chemical substances.^[ref]

An illuminating comparison is to cut two sections of one oriented composite, one across its long direction and one along it. Their patterns can differ even if their average composition is the same. This geometrical example explains why a section's appearance must be interpreted together with its orientation; it is not a species-identification test.

The distinction between composition and organization also motivates solid-state nuclear magnetic resonance. Historical CPMAS demonstrations included an ivory billiard ball, showing how a rigid biological material could yield chemically informative spectra.^[ref]

The Schreger pattern is the recurring light-and-dark motif visible in sections of elephant ivory, the dentine of the tusk. Sections across the tusk can show a checkerboard-like motif, whereas lengthwise sections show alternating bands. This is an example of how the appearance of a three-dimensional biological tissue depends on the plane of observation.^[ref]

In2017, a study based on one elephant tusk compared several section orientations with microscopic observations and computed tomography using synchrotron radiation. The researchers related darker regions to dentinal tubules running obliquely to the cut surface and brighter regions to tubules running more nearly parallel to it. They proposed a three-dimensional tubule arrangement to connect those observations.^[ref]

The optical mechanism also involves the surrounding material: a2018 study combined X-ray scattering, polarized-light observations and scanning electron microscopy to investigate mineralized collagen orientations. Its authors proposed that the layered collagen architecture and birefringence contribute to the pattern. Thus a visible band is evidence about structure and light interaction, rather than a direct map of a single chemical constituent.^[ref]