Pyrite and gold form under similar conditions, and can even occur together in the same rock.

In fact, in some ore deposits, tiny amounts of gold can occur as inclusions and substitutions within the pyrite! This is important because even though you might think gold is better for its monetary value, pyrite can be an important clue for finding gold deposits, as well as for use in sulfur and sulfuric acid production. Sulfur dioxide is commonly used as a reducing agent in the paper industry. Pyrite crystals also have use in ornamental jewelry and other commercial applications.

Geologists can use density to distinguish between pyrite and gold because gold has a specific gravity of about 19.3 while the specific gravity of pyrite is only about 5.

Specific gravity is the density of a substance compared to water. So water has a specific gravity of 1. Density is the degree of compactness of a substance. That means that gold molecules are extremely compact, so gold is about four times heavier than pyrite. Even if the gold is alloyed, or mixed, with other metals, it will still be noticeably heavier than pyrite.

However, I said that gold and pyrite can be found together, so how will you tell them apart? Geologists have a few more tests besides density.

These tests include:
1) Tarnish. Pyrite often has a tarnished, or dull, surface, while gold maintains its bright, shiny surface.
2) Color. Pyrite is usually slightly brassy, or oranger, in color, while gold is typically a whitish yellow color.
3) Shape. Pyrite is typically angular, or pointy, and often found as a cube (6-sided) or octahedron (8-sided) shape. Gold can resemble a shape similar to pyrite, but often has rounded edges.
4) Striations. Pyrite frequently has very fine, parallel lines across the surfaces, whereas gold crystals do not.

If you don’t mind damaging the specimens a little, two more useful tests are streak and hardness.
5) Streak. Streak determines the color of the mineral if it were powdered. You can identify the streak by scraping the mineral across unglazed porcelain. Pyrite has a dark greenish-black streak, while gold has a yellow streak.
6) Hardness. Hardness is how resistant to breaking or damage the mineral is. Pyrite is a hard mineral and can scratch a copper surface. Meaning pyrite could scratch a penny. Gold, however, is very soft. Gold cannot scratch a penny. In fact, if you poked a small flake of what appears to be gold with a needle, pyrite might break while gold will bend around the point.

The density of gold is approximately 19 grams/ cubic centimeter, and the density of pyrite is approximately 5 grams/ cubic centimeter, so gold is almost four times as dense as pyrite.

A geologist who wants to compare the density of two samples could put each one in a graduated cylinder full of water and see how much the water level rises to determine the volumes. Then they could measure the mass of each sample with a balance. Dividing the mass of each sample by its volume gives the density.

Other ways to tell the difference are shape (pyrite often grows in small cubes, while gold does not) or hardness (Pyrite has a hardness of about 6, and gold has a hardness of about 2 on Moh's scale).