Fiberglass is simply glass that is in the shape of thin threads. Because of their small diameter, fibers of glass are very flexible. However, they can still be broken, and like a broken window, they are capable of cutting the body, although the cuts will be very small. Technically, fiberglass is said to be a man-made mineral fiber, as opposed to a naturally occurring mineral fiber like asbestos which is mined directly from the earth. Fiberglass is made by melting glass and drawing it out into strands.
Rock wool is another man-made mineral fiber that can be used for insulation.
It is made by melting rocks or slag and drawing it into threads. Rock wool is not encountered today as much as fiberglass although it has similar properties Fiberglass can be combined with various polyester or epoxy resins to produce such things as fishing rods, skis, sports car bodies and bullet proof vests. The combination of fiberglass and resin can be very strong. The glass tends to reinforce the resin yielding an end product that is considerably stronger than either of the original components. Raw fiberglass can also woven into fabric for use as products such as draperies.