how can one tell the difference?
I thought it might be the color.. That pyrite didn't look like the usual gold color you're used to. Looking around here's a few explanations.
Color
After you've gained some experience with gold panning and prospecting, you should easily be able to tell the difference between pyrite and real gold simply by looking at its color. Even individuals without any past experience will likely see a noticeable difference between the two minerals when they're placed side-by-side.
Pyrite has a darker yellow color to it that's similar to brass, while gold has a vibrant yellow color that's highly reflective to the surrounding light. Holding a piece of pyrite under the light will reveal a bright yellow color that's stronger than the dull yellow color of pyrite. Until you're able to easily identify gold, you should keep a piece of pyrite around in your pocket to compare your finds with.
Streak Test
Streak tests are one of the most effective methods used to check and see if a mineral is actually gold. If you don't know what these are, let me explain -- streak tests are performed by rubbing a mineral nugget on an unweathered surface known as a streak plate. Once the mineral is rubbed, it leaves behind a colored trail of mineral powder that may give away clues as to what type of mineral it is.
Pyrite typically leaves behind a black-dark green powder, while gold leaves behind a bright yellow powder. You can find streak plates to test for gold at dozens of mineral and hobby shops throughout the country, as well as online shops.
The most obvious way to tell the difference between them is the specific gravity. When panning out material, gold will settle and concentrate in the bottom of the pan, but pyrite will move freely in the pan. You will often see them at the surface mixed in with the lighter sands and gravels. Proper gold panning will easily separate the two, as the small specks of pyrite will wash out of the pan while the denser gold particles will be retained.
The color is also a good indicator between the two. While gold obviously has a golden color, pyrite generally has a brassy and shiny coloration. It has shiny surfaces that catch the reflection of the sun. If you move your gold pan in a circular motion in the sunlight, gold will maintain a consistent color, while pyrite will flash in the sunlight.
If you take a pocket knife and separate the particles out, their different hardness will be readily apparent. Pyrite is much harder, so if you smash it with the tip of your pocket knife it will shatter into several pieces. Gold on the other hand is a very soft and malleable metal. It can be smashed flat without breaking apart.