An interesting article:

Gold Grain Morphology
Source: Gold Investing News, Leia Michele Toovey 09/20/2010

Gold's high value means a deposit of very small concentration can be economically mined—great news for miners, but complicating for prospectors. Gold is often found in ores composed of rock with very small particles. When it comes to mining, ore with grades of gold as little as 0.5 ppm can be economically mined. Because ore grades of 30 ppm are usually needed before gold is visible to the naked eye, gold in most mines is invisible. Not only can viewing individual grains of gold provide a challenge, the nature of prospecting means finding these grains through conventional sampling is nearly impossible.

Prospectors, typically, use geophysical data to narrow down a region of interest, then return to complete sampling and assaying. Prospectors take only a few samples in an entire region to determine it warrants more exploration. Thus, it's easy to miss an economical gold deposit. Working on such small scales is problematic because it is very easy to completely overlook an economic deposit. So how do prospectors improve their odds? Some techniques enhance the chances of finding gold. Gold grain abundance and grain characteristics have been applied systematically in the past 35 years. The most common characteristics used are size, shape and chemical composition.

Gold grain morphology is an example of one of these techniques. Gold grain morphology takes advantage of the fact that gold is very malleable, and the surface shape of gold will change as the metal travels farther away from its source. Under the gold grain morphology classification system, gold grains are rated as either pristine, modified or reshaped. Pristine gold grains have maintained their primary shapes and surface textures. Discovering pristine grains indicates you're less than 500m to the source of the gold sample. Modified gold grains are slightly reshaped, the discovery of which indicates you are 0m–1,000m away from the source.