The story of glass begins from the time that human beings discovered the fact that sand, at high temperatures, becomes molten to form a non-crystalline substance. This had the ability to become solid and hold its shape after being exposed to temperatures beyond 1450 degrees centigrade. The earliest known glass was formed this way (naturally and by design) and continues to do so till today. Glass substances like Obsidian, fulgurites or moldavite can occur naturally in certain conditions; for instance, Desert glass can be formed from the silica in sand as a result of lightning or meteor strikes. Around 3500 B.C. is the date to which glass as a medium of aesthetic expression is traceable. The invention of glass has been attributed to the Egyptians, Mesopotamians and Romans.