What is a natural material composed of masses of mineral crystals known as?

The correct answer is "rock" because a rock is defined as a natural material made up of solid aggregates of mineral crystals or grains. Rocks can be composed of one type of mineral or a combination of different minerals, and they form through various geological processes, including cooling of molten magma, sedimentation, and metamorphism.

Granite is a specific type of rock that is primarily composed of quartz, feldspar, and mica, but the question seeks a broader definition. Sediment refers to particles that are transported by and deposited through various processes, while ore specifically relates to rocks that contain valuable minerals for extraction. Therefore, "rock" encapsulates the concept of a natural material composed of mineral crystals, making it the most accurate choice.