Hi G. Wilson,
Actually, this one seems to be an old wive's tale. That cracking or popping sound your hear happens because the gases in the synovial fluid are released (synovial fluid is a natural lubricant for your joints. The gases are just run-of-the-mill oxygen, nitrogen and carbon dioxide).
Generally, cracking your knuckles doesn't cause any harm to the joints.
Butif it hurts when your knuckles pop, then you should see a doctor. Some studies show that frequent or repeated knuckle cracking may damage the soft tissue of the joint, which could lead to swelling and a weakened grip.Rheumatoid Arthritis, on the other hand, can cause significant joint deformity, dramaticallly changing the appearance of your fingers.
Knuckle cracking does not change the appearance of your fingers. That being said, I would not suggest it if you have previously been diagnosed with damage in your joint or to the fluid in between. If there has been previous damage to your joints or the fluid, it can cause swelling and even discomfort or pain. Good luck and best wishes!
Further to Joy's answer, the gas released is mainly argon which is the same gas used in light bulbs and is totaly harmless.