obsidian

Everyday

1. Obsidian is a hard, shiny black (or dark) natural glass that forms when lava from a volcano cools very quickly. tree →
2. Obsidian was used long ago to make very sharp knives, arrowheads and mirrors. tree →

In the lessons

In science lessons on rocks, obsidian is an igneous rock (made from cooled lava) that cooled too fast for crystals to grow, so it is glassy; when it breaks it has very sharp curved edges. In history, the Aztecs and other peoples made tools and mirrors from it. In the game Minecraft, obsidian is a very hard block. Phrases: «obsidian blade», «volcanic glass».

Examples

• Obsidian is black volcanic glass. tree →
• Obsidian forms when lava cools very quickly. tree →
• Ancient people made sharp arrowheads from obsidian. tree →
• The museum had an Aztec mirror made of obsidian. tree →
• In Minecraft you need obsidian to build a portal. tree →
• Obsidian is an igneous rock. tree →

Note

Uncountable: «a piece of obsidian». Pronounced ob-SID-ee-un. Safety: broken obsidian is sharper than a knife, so handle with care.