Veins of extremely coarse-grained igneous rocks are called pegmatites.

Here's why:

* Pegmatites are igneous rocks with exceptionally large crystals (typically over 1 centimeter in size).

* They form in the late stages of magmatic crystallization, often in the margins of intrusions.

* The large crystal size is due to slow cooling rates and the presence of abundant water and volatile elements in the magma.

While veins of other igneous rocks can be coarse-grained, the term "pegmatite" specifically refers to those with exceptionally large crystals, making them stand out.