Answer: Trichilemmal Cyst (aka Pilar Cyst)

Technically they are different than a sebaceous cyst (Medscape on the specifics). Originating from the outer root sheath of a hair follicle they present as a nodular subcutaneous mass.It contains a large amount of keratin which calcifies giving it this classic appearance. 90% are in the scalp.

Historically a benign lesion, there have been reported cases of malignant transformation (seriously!), so you one may want to keep an eye on it for enlargement or look for metastatic adenopathy.

Reference: http://www.ajnr.org/content/22/1/180