It is a science question to help my daughter study

Simple...

Crystals take time to grow. First, you can't have crystals without heat. Metamorphic rocks often have crystals, but never large. This is because they are only heated to a plastic type state. This allows minimal movement of minerals that can be crystallized. Igneous rocks are the greatest source of crystals. You can see a lot by the crystals in it. The type of mineral crystals can indicate where in the world it came from. The size can tell you how long it was molten. Sometimes molten masses can be created and cooled under ground, allowing for long cooling times and larger crystals. Other times an eruption of a volcano can create rapid cooling times and smaller crystals. Especially large crystals can be found in mantle lava. Some of the lava from the mantle has been under constant heat for thousands of years.