Lava rocks can have a range of grain sizes, from very fine to quite coarse. It depends on the type of lava and how it cooled.

Here's a breakdown:

* Fine-grained lava rocks: These are formed when lava cools quickly, like when it erupts into water or flows over a cold surface. This rapid cooling doesn't give crystals much time to grow, resulting in small grains. Examples include basalt, andesite, and rhyolite.

* Coarse-grained lava rocks: These form when lava cools slowly, allowing crystals to grow larger. This happens when lava flows deep underground or in thick, insulated layers. Examples include gabbro and granite, although these are technically not lava rocks as they form from magma.

* Porphyritic rocks: These have a mix of large crystals (phenocrysts) embedded in a finer-grained matrix. This happens when the lava cools slowly initially, allowing large crystals to form, then cools more quickly later, forming the finer-grained matrix. Many lava rocks, like andesite, can have porphyritic textures.

So, while some lava rocks have large grains, many others have smaller grains, and some have a mix of both. It all depends on the cooling rate and the specific type of lava.