I recently made beef stock with marrow bones (sometimes called pipe bones) and in the process rendered out a heck of a lot of beef fat (tallow) from inside the bones.
Two birds with one stone, and all that.
See all the white stuff inside the bones? That’s marrow. It’s either a great culinary delicacy and the reason mankind started using tools in the first place, or totally super gross, depending on who you talk to. It’s composed of about 90% fat.
After roasting and days – days– of simmering, the bones were empty and the stock made from them was covered with a think layer of fat.