Streak is the color of a powdered mineral.  Though you would not want to powder a gem, you can test gem rough (uncut material) by using a porcelain tile.  The porcelain tile has a Mohs hardness of about 6.5, so many of the less important gems will leave a streak of powder behind if you move them over the porcelain.  This is analogous to writing with chalk on a blackboard.

Opaque gems and metals are best tested this way.  Gold will leave a golden streak, but fool's gold, the mineral pyrite, will leave a black streak.

Harder precious stones such as quartz, tourmaline, most garnets, peridot, beryl (aquamarine, emerald, etc.), corundum (ruby & sapphire), Chrysoberyl (alexandrite & cat'seye), and diamond will not leave a streak.  They are too hard.  This is a good thing because hardness is part of durability.