It is purported that after a person is cremated (specifically a holy person, or such as someone with a high meditative attainment) there are often left behind pearl-like relics. You can read about them here: https://en.wikipedia.org/wiki/%C5%9Aar%C4%ABra

As for how this can happen, there is evidence that under certain conditions of heating, human bones can form crystalline structures. But I don't think that the process is fully studied or understood.