Determining the difference between antique marbles and new marbles isn't as easy as it may seem at first glance. During the 20th century, millions of new marbles were made to look like old marbles. There are an innumerable number of factors that come into play when evaluating marbles. The best way to tell the differences is to study marble history, the categories of marbles, the unique characteristics of each type and then to use that knowledge to make educated evaluations and decisions. In a 2009 interview with "The Collector's News," marble expert Alan Basinet summed up the dilemma regarding old and new marbles: "It's not like collecting coins where you pretty much know everything that's out there and how much was made. With a coin, you can also tell exactly the year it was made. I've been doing this for more than 10 years and collecting for an additional five, and I probably don't have a week go by that I don't see a marble that confounds me. I don't think I will ever see every variation that's out there."
Get out your collection of marbles and study each marble carefully.
Separate marbles into groups of hand made or machine made. To determine if they were handmade, look for the pontil mark to determine how they were created. A pontil is a rough mark on a handmade marble which was caused when the marble was cut off of the cane, the glass rod they were formed on. Keep in mind that many machine-made marbles are as old as some handmade marbles.
After separating handmade from machine, separate those marbles into sub-categories---cat's eyes, swirls, etc.
Within sub-categories look for obvious similarities and subtle differences. Find two similar marbles that seem to be different. Hold each up to a light with a 30X magnifier glass.
Compare size. Many smaller marbles were manufactured specifically for use in games and are still being produced today.
Compare coloring. Antique marbles are brighter because they were made with better materials. Machine-made marbles are duller.
Compare quality of glass. Antique marbles are made from high quality glass, whereas machine made marbles are lower quality glass and shatter easily.
Once the above inspections and comparisons are made, it should be more obvious that some of the marbles are newer. Separate those and continue.
Begin to look for subtler nuances between marbles. This is where extensive research into marbles pays off. Such knowledge is only attained by serious research, for instance hands-on inspection of known historic marbles at marble shows, comparing old and new marbles at shows and talking to experienced collectors and dealers about them.
Hold the marbles up to the light and look for subtle flaws or marks that indicate the age. Older marbles will have certain bubbles or flaws caused by the glass-blowing process. Compare the colors of one type of marble to another. Newer marbles will have slightly different colors in their designs, and the designs themselves often look different. Refer to knowledge gained from marble research during this step.