Igneous
Born from fire
Made when hot melted rock cools down and hardens. If it cools slowly deep underground you get big crystals, like in granite. Fast and you get glassy rock like obsidian.
Examples: granite, basalt, pumice, obsidian
Rock of the Day
Obsidian
Obsidian is volcanic glass! When runny lava cools super fast, crystals never get a chance to grow — so it ends up smooth and shiny, with edges sharp enough that ancient peoples used it for tools.
The Three Rock Families
Sedimentary
Built in layers
Sand, mud, pebbles, and even old seashells pile up over millions of years and get squished together. That's why you can see stripes in canyon walls — each one is a layer of time!
Examples: sandstone, limestone, shale
🪨
Metamorphic
Changed by pressure
Take any rock, bury it deep, and squeeze it with enormous heat and pressure — it transforms into something new and tougher. Limestone becomes marble. Shale becomes slate.
Examples: marble, slate, gneiss, schist
Wild Rock Facts
- The oldest rocks ever found on Earth are about 4 billion years old.
- Pumice floats! It's so full of gas bubbles it can drift across the ocean.
- Diamonds form around 100 miles underground under incredible heat and pressure.
- Lots of limestone is made from ancient shells and coral — it used to be alive.
- On the Mohs hardness scale, talc is the softest (1) and diamond the hardest (10).
- A geode looks like a boring lumpy rock until you crack it open and find crystals inside.
Quick Quiz!
Start Your Own Rock Collection
- Grab an egg carton — perfect for keeping specimens apart.
- Write down where you found each rock. Location is half the story!
- Try the scratch test: can a nail scratch it? Can it scratch glass?
- Always ask permission before collecting, and never chip at protected sites.