Most modern counterfeits are made of thinly plated steel, brass, copper, or zinc alloys.  These metals are significantly less dense than pure silver or gold, which means a counterfeit made to the exact same dimensions as the genuine bar or round will be under weight.

Likewise a counterfeit made to the correct weight will be larger by volume than the genuine sample.  This is often achieved by closely matching the diameter (or length and width) of the genuine sample, and increasing the thickness.

Most fakes can be detected by measuring the dimensions and weight and comparing them to documented values of genuine samples.  This is where the detailed information in the Fake Bullion Database comes into play.

As always, it is important to use the appropriate tools for the job.  When comparing genuine samples to counterfeits, we are looking at differences of just a few millimeters or grams, so accuracy is very important.  Precise dimensions are best measured using calipers, and digital calipers are both affordable and easy to use.  Small digital scales are also affordable and portable.  Consider a more precise 100 x 0.01g digital scale for fractional bullion and one ounce bars and coins, and a higher capacity 1000 x 0.1g digital scale for bars and rounds over three ounces.  Periodically checking your scale with a calibration weight is a good idea to ensure accuracy.

One troy ounce is equal to 31.1 grams.  Genuine one troy ounce bars and rounds will weigh 31.10 grams or slightly more.  Depending on quality control, it is possible for genuine pieces to weigh a hair under one troy ounce (we have come across a few samples weighing 31.08 grams), but generally they will be equal to or slightly greater than 31.10 grams.

Counterfeit pieces range anywhere from 28 grams to 35 grams.  Any piece weighing less than 31.10 grams is immediately questionable.  Any piece weighing significantly more than 31.10 grams should also be questioned.  While legitimate manufacturers do tend to err on the side of caution, they also are not in the business of giving away free metal.

Of course weight is only half the story.  A counterfeit made from a less dense metal like brass can match the weight of a genuine piece by being over sized, so even when a piece does weigh as expected, it is important to also check the dimensions.

Be sure to wipe the jaws clean, push them together, and zero your calipers every time you use them.  When measuring the diameter of a round, or the length and width of a bar, ensure the edges of the sample are flat against the caliper jaws.

When measuring the thickness of a bar or round, do so at the edge of the piece, on the rim.  The thickness of the center design can vary depending on the striking pressure.  The rim can also vary slightly due to variations in the manufacturing conditions, but is generally more consistent than the center of the coin or bar. 

Compare the measured dimensions to those listed for the genuine and counterfeit versions in the Fake Bullion Database.  Be sure to match the year and/or version of the item you are evaluating, as designs and dimensions can change over the years.