The purity of gold depends on how much other metals or impurities, non-gold material is encapsulated within the gold metal. Black sands freely mixed like beans in a soup don't affect the purity of the gold its self, it must be physically mixed into the gold more like sugar dissolved in a soda pop to affect the purity of the gold.
You may not actually know the purity of your gold unless you have it refined or assayed.
Ounces are not ounces when it comes to precious metal, a Troy ounce, used to weigh precious metal equals about 31.10 grams. The ounces we see used most in the U.S. are like the ones used to weigh food as in super markets and equal 28.35 grams. There are about 12 troy ounces in a Troy pound, and 16 ounces in the avoirdupois pound or super market weight. A penny weight is roughly 1/20th of a troy ounce. The ounces can vary depending on what standard of measure the buyer uses to weigh your gold don't be afraid to ask them so you can do your own math.
Your nuggets, depending on size and shape may be worth more as individual nuggets vs being melted into a button or a bar, but remember someone has to pay for any refining process and it wont be the refiner buying the gold.