The difference between black nails and gray nails is mainly reflected in different colors, different causes and different treatment options. 1. Color difference: black nails are mainly manifested as black nails. Gray nails are mainly white, yellow and thickened. 2. Different causes: black nails may be related to onychomycosis and melanoma. Gray nails are caused by fungal infection of the nails, which is also called onychomycosis. 3. Different treatment programs: (1) For black nails caused by onychomatous nevus, observation is usually the main concern. For black nails caused by melanoma, surgery is usually the main treatment. (2) The treatment of gray nails is mainly based on antifungal, which includes topical miconazole nitrate cream as well as oral terbinafine tablets. As there are more differences, the specific situation needs to ask the doctor for a face-to-face consultation.