Thumb nail crescent purple consider trauma, Raynaud’s syndrome, psoriasis and so on. 1. Trauma: If the thumbnail has suffered a violent impact, it will cause bleeding under the nail, and then the thumb crescent will appear purple. 2. Raynaud’s syndrome: Raynaud’s syndrome, also known as Raynaud’s disease or Raynaud’s phenomenon, is mainly a vascular nerve dysfunction caused by small arterial spasms in the extremities, the cause of which is not very clear, and the disease is slow to start, usually after cold, especially after the finger contact with low temperature is prone to seizures, and the color of the finger becomes white and then turns into purple when the seizure occurs. 2. psoriasis: psoriasis is a chronic scaly skin disease caused by the interaction of many factors, this skin disease if it involves the nails, will cause nail lesions, mainly will appear thimble-like, some will also appear point-like depression nail separation under the nail hyperkeratosis, and some also manifested in the thumb nail crescent purple. Thumb nail crescent purple may also have other causes, it is recommended to go to the hospital, complete the examination to clarify the cause of the disease, under the guidance of the doctor to give targeted treatment or treatment.