Igneous rocks, one of the three main classifications of rocks, are formed when molten material cools, or in general, when a rock is formed under extreme heat. The rock begins as either magma, which resides underneath the earth, or lava, which has erupted from a volcano.
The rate at which the molten material cools determines most of the future characteristics of the rock it forms. If it cools quickly, the rock will feature very fine grains. If it cools more slowly, the rock will have more coarse grains.
Obsidian rock occurs when the cooling happens instantly. This typically is only possible when the molten material comes into contact with water, usually when magma or lava erupts into an ocean or lake. This creates the smooth and glassy texture, because it cools rapidly crystals that normally are formed in other igneous rocks do not form in obsidian rocks.
It is considered an extrusive igneous rock because it must come into contact with water to rapidly cool, so it must form above the earth’s surface. Igneous rocks that form below the earth’s surface are caked intrusive rocks. It’s composition is very similar to granite or rhyolite, but its properties are very different because it cools instantly, while granite and rhyolite cool fairly slowly.
Obsidian has been given the name ‘Volcanic Glass’ because of its smooth and classy texture, and its name reflects its features, and it technically really is glass and not a rock at all. Typically a shiny dark black, it comes in many different varieties, depending on its chemical makeup, especially depending on its iron and magnesium contents that give it brown or green colors respectively. It can also have “sheens”, which are bubbles within the rock that occur if air is present when the rock is being formed. Sheens add a variety of colors and patterns to rock, varying from blues, purples, golds, yellows, and rainbow effects.
Because of these characteristics, obsidian has been used by humankind in a multitude of ways. For its beautiful appears and features, it of course has been used in any kind of jewelry possible. But also it was very usable to forge weapons of all kinds, especially in the stone age, including spears, knives, daggers, arrowheads, and hooks. Even in modern times, obsidian is used in medical and surgical tools.
Obsidian can be found anywhere in which eruptions occur and are near or were near a water source at the time of the eruption. Obsidian flows are large areas of obsidian rock and can actually be hiked over.