Lava is on the Earth's surface so it cools quickly compared to magma in Earth.
Rocks form quickly and mineral crystals are very small. Rhyolite is one type of rock that is formed when lava cools.
It contains similar minerals to granite.
The mineral crystals are much smaller than
the crystals in the granite.
Sometimes, lava cools so fast that crystals cannot form at all, forming a black glass called obsidian.
Obsidian is not crystalline, it is not a mineral.