What type of rocks cool so quickly, that no crystals can form on its surface?

Question

What type of rocks cool so quickly, that no crystals can form on its surface?

Answer 1

Are you looking for obsidian? It's an igneous rock from magma that is compressed and it has a glassy look to it.

Answer 2

Glass, obsidian, pitchstone etc.

Answer 3

If a molten rock cools really rapidly there is not time for crystals to grow and the rock crystallises as glass - Obsidian is a natural volcanic glass.  In impact structures, sometimes you can find glassy material, called pseudotachylite, which is formed by instantaneous melting and freezing of rocks due to the heat of the impact.

Answer 4

obsidian and other glasses.