​Explanation

• ​The streak of a mineral is the color of its powder and is a way to identify different minerals. Hardness is judged by scratching other minerals. Luster is the amount of light that is reflected by the surface of the rock. Remember that when a rock is rubbed on the ground, it leaves a streak of color behind. This is how a streak test is performed.

​Explanation

• ​Any mineral can scratch any other mineral that is softer than itself and can be scratched by any mineral harder than itself. The Mohs hardness scale ranks minerals from soft to hard, and this characteristic is one way of identifying minerals.

​Explanation

• ​The correct answer is “the rate at which the minerals cooled.” Crystal size in mineral formation depends on the rate of cooling of lava or magma. The materials on the surface or up high in the crust cool faster with little time for crystals to grow, creating small crystals. Lava and magma that cool slowly tend to form large crystals. The amount of materials do not affect crystal size, nor does the kind of mineral forming.

​Explanation

• ​The correct answer is “high temperature and high pressure.” Diamonds are formed deep in the mantle, with high temperatures and high pressure. The extreme pressure and temperatures alter the graphite and form diamonds. The depth is part of what causes the structural changes.