Obsidian is a volcanic glass that forms when lava cools very quickly and does not have time to form the crystals common to most igneous rocks. It can be worked to make very sharp tools, but it is fragile and needs to be re-sharpened often.In the past, obsidian was used to make knives, arrowheads and blades. It was also used for decorative items such as beads, pendants and carved figurines. The Aztec used obsidian to make such items as razors and mirrors.Obsidian was also a valued trade item in Mesoamerica. Ancient trade routes can be tracked because each lava source has a unique chemical composition. Archaeologists can analyze the obsidian to identify its source, and then calculate how far certain tools traveled. This is very important because it helps archaeologists understand how past cultures interacted. Today obsidian is sometimes used to make surgical blades, because it can create an edge much sharper than surgical steel. This results in finer incisions and leaves much smaller scars.