Answer the following question given this paragraph: 

Many people think of bones as being dead, dry, and brittle. These adjectives correctly describe the bones of a preserved skeleton, but the bones in a living human being are very much alive. As shown in Figure below , the basic structure of bones is bone matrix , which forms the underlying rigid framework of bones, formed of both compact and spongy bone. The bone matrix consists of tough protein fibers—mainly collagen—that become hard and rigid due to mineralization with calcium crystals. Bone matrix is crisscrossed by blood vessels and nerves and also contains specialized bone cells that are actively involved in metabolic processes.


Q: The bone matrix consists of tough protein fibers—mainly what?


A:
collagen