Which would be LEAST useful in identifying a mineral crystal?

In the context of identifying mineral crystals, size is often the least useful characteristic. While size can provide some information about the environment in which a mineral formed, it does not consistently aid in the identification process. Minerals can occur in a wide variety of sizes, even if they share the same crystal structure or composition.

On the other hand, shape, color, and hardness are much more reliable for mineral identification. Shape refers to the well-defined geometric forms that crystals can exhibit, which is often a distinctive property of a mineral. Color can give clues to the mineral's identity, although it can vary due to impurities and conditions of formation. Hardness, measured by its resistance to scratching, is a key identification characteristic based on the Mohs scale, providing a quick way to differentiate between various minerals. Therefore, while all characteristics can aid in identification, size has the least significance compared to the others.