Glass forms when a mixture of sand, soda and lime is heated to a very high temperature into a liquid molten state. When it cools, it does not form crystals. So it is not a true solid material, even though it is rigid.
Fiberglass is composed of natural ingredients like sand and recycled window glass and bottles. The ingredients are melted down and spun into long strands of fibers that are woven together to form "glass wool."
Since the 1930s, it has been used primarily as insulation to reduce home energy costs, and also control noise transmissions in buildings from furnaces and duct systems. When coated with hardening materials, fiberglass can be molded into spa tubs, boats, and other shapes. Fiberglass comes in rolls, batts and also as loose insulation that can be blown into place.
There are lots of naturally occurring glassy materials. Obsidian is volcanic glass, formed when the intense heat of a volcano fuses sand. When lightning strikes a mass of sand, it can form a glass called fulgarite. Tektites are a form of glass formed from rocks when meteors strike the earth.