Here's why:

* Streak is the color of a mineral's powder. It is determined by rubbing the mineral against a streak plate (unglazed porcelain tile).

* Color can be misleading because many minerals come in a variety of colors due to impurities or weathering.

* Streak is a more reliable indicator of a mineral's composition because it is less affected by external factors.

For example, pyrite (fool's gold) can be golden yellow, but its streak is black. This helps distinguish it from real gold, which has a golden yellow streak.