A gold nugget is a big glob of gold. A gold nugget isn't a big crystal of gold; it's a bunch of little crystals that have been combined. Single naturally formed gold crystals are much, much rarer: the largest gold nugget ever found weighed 173 pounds, while the largest gold crystal ever found (pictured above) weighed just over 200 ounces. Single crystal, in this case, means that the entire element is comprised of a regularly repeating pattern of atoms, as opposed to a bunch of different repeating structures that have been mushed together.
Figuring out whether this is a real crystal is very, very tricky. It would be easy to fake by casting liquid gold into a crystal shape. In particular, the trapezohedral crystal in the picture (nicknamed "the golf ball") was pulled from auction in 2008 because of doubts about its authenticity. Traditionally, you can fire x-rays at a sample like this and analyze the resulting diffraction patterns to determine what its crystal structure is, but gold is so dense that x-rays can't penetrate more than a few hundredths of a millimeter beneath the surface. Instead, scientists at Los Alamos National Labs are using neutron imaging, since neutrons can make it a centimeter or two before being absorbed.
So far, they've proved that the two samples on the left and the right of the image above are in fact single crystals that have been deformed over time. The "golf ball" in the middle is going to take a little bit more work.
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Via Miami University