http://www.usgoldbureau.com/bullion-gold-coins - Investing in gold and silver proof coins can be a very lucrative move. This is because U.S. mint coins that have been produced as gold and silver bullion pieces only increase in value through the years. Mint state coins can fetch even higher prices if you find the right dealer. What is a proof coin, you ask? It is actually a coin that is produced with special qualities, unlike the usual coins that you see.