Several methods exist to identify minerals, including the streak test, the Mohs hardness scale, the fracture test and the acid test. Based on these tests and the mineral's physical appearance, scientists can identify what the mineral is. Other methods, such as testing for specific gravity, require special instruments. Many of these methods are used in conjunction with one another.
The streak test involves taking a sample of the mineral and rubbing it against a non-glazed porcelain plate, then examining the residue. Two minerals can be the same color but leave a different streak; for example, gold has a yellow streak and pyrite has a greenish-black streak.
The Mohs scale of hardness involves scratching one mineral against another to see which is affected.The scale consists of 10 grades, with talc as the softest mineral and diamond as the hardest.
The fracture test involves breaking a sample to determine its cleavage. The molecular alignment of minerals determines how they cleave, with many minerals having several directions of cleavage. They can make cubic, rhomboid, octagonal or dodecahedronal crystals. Quartz, however, simply breaks along a random plane.
The acid test is used to determine whether a sample is a carbonate or sulfide by observing its reaction to hydrochloric acid. Carbonates bubble and release carbon dioxide. Sulfides emit a characteristic sulfuric odor of rotten eggs.