Is obsidian a mineral? The answer might surprise you

So is it like window glass?

Essentially yes, though with some nuance. Both are amorphous solids with no crystalline structure. The difference is that window glass is made by human hands, while obsidian is made by a volcano at temperatures between 700 and 1,200 °C. The process is exactly the same: melting and rapid cooling.

Interestingly, synthetic glass has existed for around 5,000 years. Obsidian has been used by humans far longer: there are knapped obsidian tools dating back more than 700,000 years.

Obsidian facts you probably didn't know

• The sharpest edge in the world. A well-knapped obsidian blade can have an edge just 3 nanometres thick — around 500 times sharper than a surgical steel scalpel. For this reason, some specialist surgeons still use it today in eye microsurgery.
• Weapon, mirror and oracle. The Aztecs used it simultaneously as an arrowhead, as a mirror for divination rituals (tezcatl), and as a sacred symbol of the god Tezcatlipoca, «the smoking mirror».
• A geological clock. The hydration layers that form on the surface of obsidian over time allow archaeologists to date objects with considerable precision. The technique is called obsidian hydration dating.
• There is obsidian on other planets. Volcanic glasses similar to obsidian have been detected on the surface of Mars, where intense volcanic activity also took place.
• It's not always black. Although black obsidian is the best known, its colour depends on impurities: magnetite particles make it black, iron oxides tint it red or brown, and when it contains small gas bubbles or mineral inclusions, the «snowflake» effect appears — or the golden and silver sheen of rainbow obsidian.