High Quality Content by WIKIPEDIA articles! Natural holmium (Ho) contains one stable isotope, 165Ho. A number of synthetic radioactive isotopes are known, the most stable one is 163Ho, with a half-life of 4,570 years. All other radioisotopes have half-lives not greater than 1.117 days in their ground states (although the metastable 166mHo has a half-life of about 1,200 years), and most have half-lives under 3 hours.