Sand and gravel don't fit the definition of a mineral because they are aggregates, meaning they are composed of multiple different minerals and other materials. Here's a breakdown:

* Minerals are naturally occurring, solid, inorganic substances with a specific chemical composition and a defined crystal structure.

* Sand is primarily composed of tiny grains of quartz (silicon dioxide), but it can also contain other minerals like feldspar, mica, and even fragments of shells and other organic material.

* Gravel is larger than sand and typically includes a mix of rounded pebbles, rock fragments, and even small cobbles. The composition of gravel can vary significantly depending on its origin.

Therefore, neither sand nor gravel has a consistent chemical composition or defined crystal structure, making them not qualify as minerals. They are considered rock fragments or sediments formed by the breakdown of existing rocks and minerals through weathering and erosion.