Final answer:
The question seems to concern the study of geology, focusing on the identification of a mineral or metal based on its physical properties such as luster, color, streak, and ability to conduct heat and electricity.
Step-by-step explanation:
Based on the properties provided, such as being harder than glass, possessing a metallic luster, having a greenish-black streak, and a brass-yellow color, the subject appears to relate to the identification of minerals or rocks, commonly studied in geology. These characteristics, including being shiny, malleable, ductile, and a good conductor of heat and electricity, suggest that the material in question could be a type of metal or mineral.
Additionally, variable hardness and melting temperature are indicative of different minerals and metals. Terms like 'shiny', 'malleable', 'ductile', and 'conducts heat and electricity well' are typical of metals. These features are useful for identifying specific minerals or categorizing them for further study or application. For example, the description could match the mineral pyrite, known for its metallic luster and brass-yellow color, often referred to as 'fool's gold'.
The information provided about other materials such as mixtures of quartz, feldspar, and rock fragments, properties such as being dark, smooth, and breaking easily, or having a dark, oily smell, characterizes different types of rocks or minerals, which are integral to the study of geology.