A diamond may be forever, but that doesn't mean it has to remain with the same owner forever.
The most wonderful quality about diamonds is that they are one of the only types of physical matter on this earth that do not wear and tear. As a result, you can not tell the difference between a diamond worn 300 years ago from a diamond freshly mined. So, when you are looking at diamonds in the jewelry store, you can't tell if that diamond is "used." In fact, 1 in 10 diamonds, possibly even more that you see in jewelry stores are pre-owned stones. With that being said, chances are, the diamond you are wearing on your finger has been worn before.
Nearly every diamond that has been mined is still in circulation today. Diamonds are often sold back to dealers and retailers and are then recut, reshaped, reset, repolished, and remounted into beautiful new settings and sold as "new" diamonds. Additionally, many jewelers take used diamonds and convert them into new fashionable designs making the tracking of these used diamonds more difficult. More than a billion dollars worth of recycled diamonds is sold in stores today. But, don't get alarmed, no one can really know for sure if you are wearing a used diamond unless you purchase it straight from the diamond mine yourself.
Being in the diamond business, I can tell you that diamonds are sold every single day to dealers around the globe and those stones are not replanted back into the ground. They are sometimes put into lots and then sold off to buyers and representatives that handle the buying for large retail companies. Other times, these lots are sold to networks of diamond buyers that sell to independent, local jewelers.
So, what's the big deal anyways? Diamonds have always been passed down from generation to generation for centuries, which we call estate jewelry, or heirlooms. But even more importantly, diamond mining causes significant environmental consequences on the environment, so you should be happy to be wearing something "used" or even request and search for something you know is used. The massive environmental disruption from mining causes soil erosion, deforestation, and ultimately the collapse of entire ecosystems. By wearing jewelry that is recycled or used, heirlooms or vintage jewelry, you are doing something that is both socially and environmentally responsible. There are literally enough harvested diamonds to satisfy all our jewelry needs for the next 50 years or so without having to mine a single stone and there is a good chance these precious gems will end up right back into the marketplace as the number of recycled diamonds is growing. And, the more recycled diamonds that are sold back to the industry, the less new diamonds need to be mined, mitigating the environmental impact.
So the answer remains, yes you are most likely are wearing a used diamond. Your stone was probably mined decades or hundreds of years ago and someone else has owned your diamond before you. Used diamonds are not in any way negative. Quite the opposite. Diamonds are durable enough to have history and jewelry that comes with history and character sets it apart from all others, making them unique. And let's not forget that these recycled diamonds provide a way to obtain beautiful pieces while knowing your gem was acquired without damaging the environment. Whatever the case, a diamond is a diamond and no matter if it's used or not and will always look brand new whether it was from 200 years ago or today.