What is a mineral's streak?

• A. The color of a mineral's surface when viewed in daylight.
• B. The mineral's luster on a polished surface.
• C. The color of a mineral's powdered form when rubbed on a streak plate.
• D. The color of a mineral's powdered form under UV light.

Answer: (C)

The main idea here is how a mineral’s true color is assessed through its powder. The streak is the color of a mineral’s powder produced when you rub it on a streak plate, usually a rough white porcelain tile. This test gives a more consistent color than the mineral’s surface, which can be stained or altered by impurities, weathering, or surface weathering. Because the streak reflects the mineral’s intrinsic color in powder form, it’s a reliable clue for identification, especially when minerals look similar on the outside but reveal different powder colors.

The other descriptions refer to different properties: surface color seen in daylight isn’t the streak, luster is about how the surface reflects light, and color under UV light relates to fluorescence rather than the powdered color.