Have you ever seen a movie or television show in which a hero saves a town from sure destruction by stopping a lava flow from an exploding volcano? If so, the hero in the show may have done so by creating a cooling barrier to turn the molten magma into a solid rock.
While many such television shows and movies about volcanoes may have many details that aren't exactly supported by science, it is true that molten magma can be cooled to form a solid. In fact, sometimes molten magma comes into contact with the air or water on Earth's surface and cools so rapidly that its atoms don't have time to form a regular crystalline structure.
When that happens the molten magma forms an igneous rock known as obsidian. Obsidian is quite unique due to its smooth, uniform volcanic glass texture. Obsidian is usually considered an extrusive rock, because it usually solidifies above Earth's surface where the edges of a lava flow come into contact with cool air or water.
When most people think of obsidian, they think of a deep black, glassy stone. Black is the most common color of obsidian, although it can also often be found in shades of brown and green. If trace elements are present in the rock, it might also take on rarer shades, such as red, orange, yellow, or blue.
Obsidian can be found all over the world in areas with volcanic activity. For example, significant deposits of obsidian can be found in the United States, Canada, Mexico, Guatemala, Argentina, Chile, Greece, Hungary, Italy, Iceland, Russia, New Zealand, Japan, and Kenya. In the contiguous United States, you'll only find obsidian west of the Mississippi River in states such as Arizona, California, Idaho, Nevada, New Mexico, Oregon, Washington, and Wyoming. Obsidian can also be found in Hawaii and Alaska.
One of the most unique features of obsidian is how it breaks. Geologists call the way obsidian breaks a conchoidal fracture. That means obsidian breaks into pieces with curved surfaces that are razor thin and extremely sharp.
This feature is why one of the first primitive uses of obsidian was as a cutting tool. Over time, ancient peoples learned to break obsidian into tools of various shapes. All sorts of prehistoric artifacts made from obsidian have been found by archeologists, including knives, arrow heads, spear points, and scrapers.
Some historians believe obsidian may have been the first material actively mined and used to manufacture large amounts of sharp tools to be used for trade. Some ancient peoples may have transported their obsidian goods thousands of miles to trade for other goods and services.
Obsidian's glassy texture also made it a good source for the earliest mirrors. It probably did not take ancient peoples long to discover that its glassy surface could be shined to create a reflective surface suitable as a mirror.
Today, obsidian still has important uses. Its smooth, glassy texture makes it popular as a raw material used to make jewelry. From reflective beads to interesting gemstones, obsidian makes beautiful jewelry. Unfortunately, it is not very hard, which makes it easy to scratch and break.
Perhaps the most popular modern use of obsidian is still as a cutting tool. Yes, we've come a long way from the Stone Age, but modern scientists have discovered that obsidian's unique properties still make it an ideal material to make surgical scalpels that have cutting edges that are thinner and sharper than the best surgical steel!