This patient has a pilomatricoma, aka a calcifying epithelioma of Malherbe, which is commonly a solitary but mobile growth under the skin which is most often confused to be an epidermoid cyst. These are benign growths, not painful or uncomfortable. I was suspicious it was a pilomatricoma when I removed it because of it's firmness, but the diagnosis was confirmed when we sent the tissue for histopathology.
Under the microscope, it's typical to see "shadow cells", pink circular areas with a central clearing/ lightened area. There is often calcification (calcium deposition) and even ossification (bone deposition) which gives this growth a very hard firm appearance.
Treatment is surgical excision as I do here, but it is a benign process and is in no way life threatening.