Why is luster can be determined simply by observation?

Luster can be determined by observation because it refers to the way a surface reflects light, which is visually perceptible. Different materials exhibit varying degrees of shine, ranging from metallic and glassy to dull or matte finishes, making it easy to categorize them based on appearance. This property is evident without the need for complex tools, allowing for quick visual assessments in mineralogy and gemology. Consequently, luster serves as a fundamental characteristic for identifying and distinguishing materials.