What is the characteristic of minerals referred to as 'luster'?

Luster is defined as the way a mineral reflects light, which determines how shiny or dull it appears to the eye. This property is crucial in mineral identification, as it describes the surface quality of the mineral under light. Luster can range from metallic, which gives a shiny appearance similar to polished metal, to non-metallic types such as glassy, pearly, or dull. Each type of luster provides valuable information about the mineral's composition and structure.

The other concepts mentioned, such as color, reaction to acids, and hardness, serve different purposes in mineral identification. Color can vary due to impurities and may not be consistent across samples. The reaction to acids relates more to the chemical properties of the mineral rather than its physical appearance. Hardness, which measures a mineral's resistance to scratching, is another distinct property that helps classify minerals but does not relate to how they reflect light. By focusing on the way light interacts with a mineral's surface, luster becomes a key characteristic in the study and identification of minerals.