Tell them that each has a characteristic color when rubbed on a streak plate, one streaks grey-black, while the other has a rusty brown streak. Have them streak the samples and hold up the black-streaking mineral. This is pyrite, also known as fool's gold. The rusty-streaking mineral is hematite, an ore of iron. Follow a similar procedure with other minerals.
Some minerals have specific characteristics.
This activity can be extended by following a similar procedure for rocks. You can begin by telling the students they have some minerals and rocks in their paper bags.