card
247

obsidian

OLogy Series
geology
card
247

obsidian

OLogy Series
geology

Obsidian doesn't look like your typical rock. In fact, it looks more like dark glass than a rock you'd pick up in your backyard. That's because obsidian is volcanic glass. It forms when volcanic lava cools so quickly, there's no time for crystals to grow. This gives the rock a black, smooth, glossy shine. Since obsidian forms from magma, it is an igneous rock.

Obsidian is:

formed in the ocean

formed underground

erupted out of volcanoes

Are you right?

Correct!

Obsidian is an extrusive rock that erupted out of a volcano. Most volcanic rocks are rough and jagged because they contain gas bubbles. But obsidian is smooth because it contains mostly of glass.

Obsidian breaks apart like glass, with smooth, curved shapes that have very sharp edges. This is why ancient cultures used obsidian for:

cutting tools

weapons

both

Are you right?

Correct!

In ancient times, obsidian was used for cutting tools and weapons, like blades and arrowheads. Well-crafted obsidian blades can be as sharp as surgical steel scalpels. Today, obsidian is used as a gem or ornamental stone.

Obsidian lava erupts as big piles of volcanic rubble. It creeps so slowly along the side of the volcano that you hardly notice it was moving.

Obsidian is always completely black.

Fact
or
Fiction
?

Fiction

Obsidian can be black, but it can also be red, brown, gray, green, and even clear. "Snowflake obsidian" contains tiny white crystals.

Definition: volcanic rock that forms when lava hardens too quickly for crystals to form
Type: igneous
Appearance: shiny, bigger pieces appear black
Texture: smooth, like glass
Cool fact 1: In the rarest occasions obsidian can be found in mahogany, rainbow, gold, and green.
Cool fact 2: Some ancient cultures used obsidian mirrors as a divinity tool because they look like crystal balls.

Image credits: courtesy of AMNH; Rosamond Kinzler: AMNH.