Final answer:
A geologist determines the metallic luster of a mineral through tests such as the streak test and the acid test.
Step-by-step explanation:
When a geologist refers to a mineral as metallic, they are describing the mineral's luster. Luster describes how light interacts with the surface of a mineral. Metallic luster means that the mineral looks like a metal, while non-metallic luster means it does not look like a metal.
To determine the metallic luster of a mineral, geologists can use a variety of tests. One common test is the streak test, which involves scratching the mineral across a ceramic plate to observe the color of its powder. Metallic minerals often have a distinct streak color.
Another test that can be used to identify metallic minerals is the acid test. Gold, for example, can be identified by treating a sample with nitric acid and observing its reaction. Gold will not dissolve in nitric acid, but it will dissolve in a mixture of nitric acid and hydrochloric acid.