Confined within each devil fruit is the spirit of a devil. Once a person consumes the fruit, the devil's spirit will be transferred to the person giving him new powers and abilities. It is said that only one person can have the power of the same fruit at a time. Also, a person can only have one devil fruit powers.
How wide is an 8mm ring? An 8mm ring is as about the width of four nickles stacked together. A standard U.S uses nickle measures 1/16 of an inch, which is almost 2mm wide. Four nickles, or 8mm, would measure 1/4 inch.
5/16 inch8mm = 5/16 inch.