According to WebMD, purple fingernails are commonly caused by bleeding under the nails; this condition is referred to as a subungual hematoma. This condition often occurs due to accidents that cause trauma to the hand, such as slamming fingers in a car door.
The most common symptoms of a subungual hematoma are severe pain and a darkening of the affected nail. Pain is the direct result of the pressure that occurs when the blood pools between the fingernail and the nail bed. It is important to consult with a physician if symptoms of a subungual hematoma are present to determine if broken bones or fractures are present.
According to Medscape author Oliver Mayorga MD, subungual hematomas can be treated with a procedure called nail-bed trephination. A physician uses a sterilized needle to lightly penetrate the nail bed until a small hole forms, allowing the pooled blood to drain out of the hole.
WebMD states that in certain cases — no nail-trauma history — tumors can cause the nail to turn purple in color and mimic the visual symptoms of a subungual hematoma. With the latter discoloration of the nail dissipates following treatment or as the nail grows out, while a tumor under the nail remains in the same area regardless of nail growth.