First of all, the streak of the two minerals is different. "Streak" refers to the color of a mineral's powder. The color of a mineral's powder can be tested by rubbing the mineral on a surface of hard white porcelain. The color of the residual streak indicates the contents of the minerals. Real gold has a yellow streak, while Fool's Gold has a black streak.
In addition to the color of the minerals' streaks, fool's gold and real gold also differ in hardness. Gold has a low hardness of 2 1/2 to 3 on Moh's relative hardness scale (roughly the same as a finger nail), while fool's gold has a hardness of 6 to 6 1/2 (a typical knife blade measures at approximatel 5.5). A relative hardness of zero signifies a liquid, while a relative hardness of 10 is equivalent to a diamond.
A mineral's relative hardness can be tested by scratching. A mineral can only be scratched by a harder mineral, not by a softer one. Therefore, to test the mystery mineral specimen you can use a different mineral with a relative hardness between that of fool's gold and that of real gold. For instance, a copper penny with a relative hardness of 3.5 will suffice. Use he mystery mineral specimen to attempt to scratch the copper penny. If you can scratch the copper penny, then the mineral can not possibly be real gold because it's softer than copper. The mystery mineral can therefore be assumed to be fool's gold (or another mineral with a relative hardness that is higher than that of copper).
In the days of the Gold Rush pyrites and gold were often confused, resulting in much confusion and disappointment. This sometimes leads to the worry in this day and age that fool's gold and real gold are difficult to distinguish. But fear not, because through the above testing methods, fools gold identification is relatively simple.
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