The most common isotope of cobalt is cobalt-59 (Co-59).
Cobalt-59 is a stable isotope, meaning it does not undergo radioactive decay. It is the only naturally occurring isotope of cobalt, making up nearly 100% of the element found in nature. Cobalt has other isotopes, such as cobalt-60, which is radioactive and used in various applications, including cancer treatment and industrial radiography, but Co-59 is predominant in natural sources.