One type of glass that I use in my mosaics is Italian Smalti, which is a handmade opaque glass tile made by a family business in Venice, Italy. The glass has a thick and textured surface and comes in many vibrant and rich colors.
"Before man figured out how to craft glass, nature was already making it. When lightning strikes sand, the heat sometimes fuses the sand into long, slender glass tubes called fulgurites. The intense heat of a volcanic eruption sometimes fuses rocks and sand into a glass called obsidian. In early times, people shaped obsidian into knives, arrowheads, jewelry, and money."
Source: Bottles Up
Glass is one of the few materials that can be recycled infinitely without losing strength, purity or quality.