The Troy weight system is of measuring that originated in medieval times. Though it can be used to measure anything, it is customary to use it to weight precious metals, black powder, or precious stones.
The Troy units are not even close to the english units that they share names with. A Troy pound contains 12 Troy ounces, and a troy ounce is equivalent to 1.097 ounces or 31.1034768 grams. While these differences may seem miniscule, understanding whether you’re buying silver in troy ounces or standard ounces is important. Consider the following math:
.097 ounces of Silver at a $40/ounce price point is worth $3.88.
If you bought 200 coins, and were .097 ounces short on each one, you’d be short $776 dollars worth of metal. Make sure you’re buying Troy Ounces.