The antique engagement ring, now more attractive, is also now worth more. At present, we have the ability to produce man-made GIA Certified Diamonds of fine quality.
Most diamond rings are over three billion years old, two-thirds the age of the Earth. There are a few "youngsters", though, which are only 100 million years old.
With a 10 power diamond loupe, a gauge, the proper light and some experience you may be able to approximate the size and quality of diamonds.