The mineral test that leaves a line of powder on a plate is called a streak test.

Here's how it works:

1. Choose a streak plate: A streak plate is a piece of unglazed porcelain, often white or black.

2. Rub the mineral: Gently rub the mineral across the streak plate, applying moderate pressure.

3. Observe the streak: The color of the powder left behind on the streak plate is called the streak.

The streak color is often a more reliable indicator of a mineral's identity than its external color, as it's less affected by surface impurities.