The fineness level of bullion varies greatly. No bullion will be 100% pure, as there will always be trace amounts of impurities in them. Some bullion is deliberately made to be anywhere from 35-92.5%, since coins meant for circulation and use wear down at a slower rate when other metals like copper are added.
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What Silver Purity to Buy
When buying government coins, make sure you pay attention to purity, since this can vary widely by country, coin type and mintage dates. It's ideal to stick to coinage containing at least 80% silver, as coins with lower amounts are less desirable.
For modern bullion it is all typically .999 fineness. You will find some that is .9999 fine, but the increase in purity isn't justified by the disproportionately larger premium you will pay for it.
What Gold Purity to Buy
You won't run into as many variations of gold purity in products. Most are 90% purity to .9999 in fineness. Some coins will be more collectable and recognizable than others, but will also come with a higher premium.
Coin Sizes and Weights
When buying coins that aren't .999 or .9999 in purity, some may be larger than a standard one ounce round or coin. The reason for this is the metal content is still one ounce-worth of silver or gold. You will find this in US Gold Eagle coins, South African Krugerrands, Mexican Sterling (.925) Onzas, etc.