A: According to BBC, an endoskeleton is a bone structure located in the interior of an animal, and an exoskeleton is a bone structure located on the exterior of an animal. Each type of skeleton has unique advantages and disadvantages in helping the animal survive.
Minerals made of only one element are called NATIVE minerals (i.e., silver, gold, mercury, copper). Minerals composed of compounds are simply referred to as minerals (i.e., quartz, which is made up on silicon and oxygen).
The most common minerals are those that have a chemical composition made of the common elements found in their environment. The Earth's crust is made up of about 95% igneous and metamorphic rocks, 4% shale, 0.75% sandstone, and 0.25% limestone.