Good question but impossible to answer without more details such as how big, superficial or deep, size, color, is it tender or painful etc.
.. Did it develop spontaneously or post injury?
Without knowing more, it would be prudent to have it evaluated with a good history, exam, x-rays and then possibly an ultrasound, mri
, and/or attempt at aspiration. If the evaluation is inconclusive it should be biopsied by someone experienced with proper techiniques in extremity biospy.
Best of luck.