Final answer:
A mineral's streak refers to the color it exhibits when reduced to a powdered form by being scratched on a streak plate. This property is particularly diagnostic for metallic minerals and can differ from the mineral's actual color. Streak is consistent and not easily altered by impurities, unlike the general color of the mineral.
Step-by-step explanation:
A mineral's streak is the color of the mineral in powdered form. This characteristic is determined by scratching the mineral across a streak plate to observe the color of the powder left behind. Notably, the color of the streak might differ from the mineral's apparent color due to the different ways light interacts with the mineral's surface when it is in powdered form. Streak is considered diagnostic for metallic minerals because they often have a different powdered color than their actual color.
Streak is important because it is more consistent than the color of a mineral, which can be affected by impurities. Metallic minerals typically show this property clearly, and while many minerals have a white streak or a streak color that matches the hand-sample color, the streak of metallic minerals can be quite distinct.