Yes, it’s legal. As long as the altered coin is not being used with the intent to commit fraud (trying to use the coins as legal tender after being altered), then it is perfectly legal. Changing the entire shape of the coin by forging it into a ring is now a piece of jewelry.
I use mostly circulated coins as material. Because they have been in circulation some may have small nicks, scratches and wear. Although I always try to pick the best coins in order to make good quality rings, please be aware that there may be minor flaws in the ring because of this.
There is always a slight taper associated with double sided coin rings. This is due to the outer edge of the coin being thicker than the center of the coin. When I measure the ring, I determine the size by the end with the smallest diameter.
Some people are sensitive to non-precious metals worn against their skin. I apply a wax coating and two layers of a clear sealer to each coin with nickel, copper, brass or bronze, which should solve any issues with allergies. This will however help only for a while, because due to the wear and tear that rings go through, the coating will eventually wear off. If you have sensitive skin I recommend having the ring made from a silver proof coin. I also silver plate all the silver coins, so they will have an extra layer of protection as well.
If the coating is coming off, you can easily reseal it yourself (e.g. nail polish, lacquer, ProctectaClear) or bring it to me and I’ll do it for you.
Basically the best care for your coin ring is to keep it dry. The silver care instructions apply to the coin rings as well. Remove before cleaning, working out or swimming to keep it from reacting with the chemicals in cleaning products or the pool.