White streak
There are hundreds of minerals that have a white streak. White, colourless, or light coloured minerals usually have a white streak, like halite (table salt) and talc (used to make talcum powder). But minerals that are highly coloured can also have a white streak. Fluorite can be purple, blue, green, yellow and pink in colour, but will always have a white streak.
It may be difficult to see the streak of hard minerals like corundum or diamond - they are not very easy to powder, but both have a white streak.