Grainy rocks have more air spaces. Grains are rounded and don't fit together well. Crystals tend to grow into each other leaving no gaps.

The homophone for "grainy" is "grainey." Both words sound the same but are spelled differently.

A homophone for grainy and rough is "graney" and "ruff." These words sound the same but have different meanings.

less than a drop of water in the chocolate to be grainy.

I am pretty sure grainy rocks because they are more common than crystals. They are more likely to contain sea creature fossils. They are likely to have sea creature fossils in them because the land we stand on now, was once covered in water.