Forgeries of ancient coins are believed to be as old as coinage itself. Thus, learning how to authenticate coins in one’s ancient coin collection is also a timeless skill, one that requires knowledge, experience/exposure, and research. Often a visual inspection will do, but sometimes more sophisticated tests are done in difficult cases.
A genuine coin, for starters, is made of the correct type and density of metal. While a silver dime is made of silver, a gold dollar is made of at least 14 karat gold. Coins that are produced completely of the right metal or alloy, too, will differ both in weight and density. Genuine coins have a standard mass measurable with a good quality balance, while each metal or alloy has a standard density measurable by simple tests.
For more valuable coins, it is advisable to use a professional grading service. Companies such as the Professional Coin Grading Service offer these services. Dealers also usually offer grading services, although they also outsource the job to the kind of companies mentioned above.
Avoid ancient coin sources, too, that say they know nothing about ancient coins but are guided by a “knowledgeable” friend or have “found it recently in granny’s attic.” There could be sellers offering enticing coins such as Sicilian tetradrachms, of relatively high grade and wonderful listing details. They could also say, however, that while the coins were from a decades-old collection and are believed to be genuine, they are unable to guarantee the authenticity – and they offer no returns.
If it’s too good to be true, it probably is. Learn authentication as an exciting part of the process, where technology and free internet have made it so much easier and convenient.