Horn vs. antler vs. ivory vs. bone

An elaboration on the important, but sometimes subtle distinctions in an article at the Pitt Rivers Museum:

Although at first sight ivory, bone, antler, and horn might appear difficult to distinguish their intrinsic qualities vary.

Strictly speaking, the word ivory only applies to the elephant tusks. However, it is generally used more widely to describe the dentine materials of other animals as well... The layers of dentine within the tusk form a wavy, interlacing pattern (or 'grain'). This offers different surface effects and also gives ivory its strength, making it suitable for long-lasting, detailed carving. An oily substance within the pattern's cavities helps reduce brittleness and give a smooth finish that can be enhanced with polishing to reveal a range of colours...

Bone refers to the hard parts of any vertebrate skeleton. Unlike ivory, which is protected by a smooth enamel layer, the surface of bone naturally appears rather grainy and coarse...

Antlers are outgrowths of bone... Cows, goats, and sheep, amongst other animals, carry horns rather than antlers. Unlike antler, horn is formed by modified skin tissue and is therefore naturally quite soft and flexible. Periodically new layers of tissue are added to the base of every horn. The material has a fibrous structure and can therefore be broken down into very thin translucent sheets...

And some additional comments of varying quality in a Natureismetal discussion thread.