Onychocytic matricoma is a rare tumor of the nail that is generally benign. Affected people often experience thickening of the involved portion of the nail. The tumor may be pigmented (melanonychia - a black or brown pigmentation of the normal nail plate) or non-pigmented. The exact underlying cause of onychocytic matricoma is currently unknown. It generally occurs sporadically in people with no family history of the condition. Treatment generally consists of surgical excision.