An igneous rock with large crystals probably indicates that the rock formed deep within the Earth, since it is typically warmer deep inside the Earth than near the surface. And some magma cools so quickly that no crystals form; we say that these have a hyaline texture (from the Greek "hyalis" meaning glass).
Hereof, why do igneous rocks have crystals?
If the magma cools slowly, large crystals form in the rock. This can happen if the magma cools deep underground. Granite and gabbro are examples of this type of rock. They are intrusive igneous rocks because they form from magma underground.
What conditions lead to the formation of large crystals is an igneous rock?
What conditions lead to the formation of large crystals in an igneous rock? Large crystals form when the magma cools slowly. You find a deposit of organic limestone.
Why is the crystal size affected by how fast the rock cools?
The activity relates to the process of igneous rock formation by the cooling of magma. It can be used to illustrate how the rate at which molten rock cools affects the size of the crystals that form within the solid rock – rapid cooling producing small crystals, slower cooling producing larger ones.