Just for the record, deer antlers are classified as a type of bone. They have a hard outer cortical layer of dense calcified bone and an inner medullary cavity (bone marrow) that has blood forming tissues and fat as wellas a trabecular bone structure.
Ivory tusks, such as elephantine ivory, are different. They are outgrowths that resemble teeth. The tusk has an outer layer of enamel but most of it is dentine.
Rhino horn is an outgrowth from the skin and is composed of densely fibrous keratin throughout.
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