What makes bones white?

Strictly speaking, bones aren't white—not gleaming white, at any rate. Living bones inside the body contain a number of pigmented chemicals. They are essential for the body's health and give our bones a slightly yellowish color. But it's also minerals that give bones their relatively whitish hue. These minerals are called calcium phosphates and they're essential to the strength and health of bones. They are held together by collagen fibers which are themselves white as indeed are the mineral crystals themselves.
Over time, the body removes old and damaged bone. Unless there is a regular supply of calcium, bones will lose their density and weaken. This is why some people take supplements of calcium phosphates as their regular diet cannot provide what their bones need.
https://www.webmd.com/osteoporosis/news/20020320/bones-need-both-calcium-phosphorus