Final answer:
The property least likely to be used by a scientist to classify a new material as a mineral is size, as minerals can vary in size. Diagnostics like cleavage, hardness, and luster are more commonly used to determine if a material is a mineral.
Step-by-step explanation:
When a scientist classifies a new material as a mineral, they are least likely to use size as a criterion. Minerals are naturally occurring, inorganic solids with a definite chemical composition and a crystalline structure. To classify a material as a mineral, a scientist would typically evaluate characteristics such as cleavage, which is the way a mineral breaks along certain planes, hardness, determined by the Mohs' scale and indicating resistance to scratching, and luster, which refers to how light reflects from the mineral's surface, including metallic and non-metallic types. Size, on the other hand, is not a diagnostic property of minerals since a mineral can grow to various sizes depending on the environmental conditions during its formation.