Californium

Cf occurs as a decay product of ²⁴⁹Bk and various irradiation processes [2]:

It has been suggested that ²⁵⁴Cf exists in supernovas. It has not been proven [2].

In pure solid form, californium exist in the following forms:

α-form: double hexagonal close-packed crystal at < 900 °C [3]
β-form: face-centered cubic crystal from > 900 °C [3]