A mole, or nevus, is a common skin growth formed when pigment-producing cells, called melanocytes, cluster together. These lesions vary widely in size, shape, and color, ranging from tan and brown to black, pink, and even purple. A sudden change in a mole’s color, especially to purple or blue, warrants careful attention. While this coloration is often due to harmless biological factors, it may also signal a more serious underlying change.
The Biological Basis of Mole Color
The perceived color of a mole is determined primarily by the depth of melanin pigment within the skin layers. Typical brown moles have melanocytes clustered in the epidermis, the skin’s outermost layer. When pigment cells reside deeper in the dermis, a visual phenomenon known as the Tyndall effect occurs.
This effect involves the scattering of light waves by dense collagen fibers surrounding the deep melanin. Shorter blue wavelengths scatter more effectively than longer red wavelengths, causing the lesion to appear blue or purplish. Therefore, a purple or blue hue indicates a change in pigment location deep within the skin, not necessarily an increase in pigment quantity.
Coloration can also be influenced by blood vessels close to the skin’s surface. A high concentration of small, dilated blood vessels creates a deep red or bluish-purple appearance due to deoxygenated blood. Assessing a purple lesion requires understanding this interplay between deep pigment and underlying vascularity.
Common Benign Reasons for Purple Moles
Many purple or blue-tinted lesions are entirely benign, arising from deep pigment or concentrated blood vessels. The Blue Nevus is a common, non-cancerous example that owes its color entirely to the Tyndall effect. These moles are typically small, round, and stable, with melanocytes deep in the dermis.
Another frequent cause for purple coloration is a benign vascular lesion, such as a Venous Lake. This is a soft, bluish-purple papule formed by a dilated venule, often found on the lips, ears, or face of older adults. The purple color comes from pooled blood, and the lesion often flattens upon compression as the blood is temporarily pushed out.
Cherry Angiomas, benign proliferations of small blood vessels, can also take on a purple shade, especially if they are larger or thrombosed. These growths start as bright red spots but darken to a purplish hue as they age, though they remain harmless. A simple sub-mole Hematoma, or blood blister, can also cause an existing mole to appear dark purple or black. This temporary condition results from trauma, causing blood to pool beneath the mole, and the coloration resolves within a few weeks.
Recognizing Signs of Malignant Change
While many purple lesions are harmless, a purple or blue-black color can be a warning sign of malignancy, particularly Nodular Melanoma. This aggressive form of skin cancer grows vertically into the deep skin layers, contributing to its dark, elevated, and sometimes bluish appearance. Nodular Melanoma often bypasses the typical radial growth phase, making it fast-growing and concerning.
Due to this rapid, deep growth, Nodular Melanoma may not follow the standard ABCDE warning signs used for flatter lesions. Instead, providers look for the EFG criteria: Elevation, Firmness, and Growth. The lesion typically feels hard to the touch and shows sustained, noticeable growth over weeks or months.
A key sign to watch for is a mole that is evolving rapidly in size, shape, color, or elevation. Malignant lesions may also present with spontaneous bleeding, crusting, itching, or pain, which are not characteristic of benign moles. Any purple lesion that appears new in adulthood and changes quickly should be considered highly suspicious.
When Professional Assessment is Necessary
Any new purple lesion or existing mole that develops a purple hue requires professional evaluation. Seek an appointment with a dermatologist if a mole exhibits warning signs, such as rapid growth, a firm texture, bleeding, or crusting. This is especially important for lesions that are elevated or growing quickly.
The dermatologist performs a thorough skin examination, often using a hand-held magnifying device called a dermatoscope. This device allows the practitioner to see patterns of pigment and blood vessels beneath the skin’s surface. If the lesion’s features are ambiguous or concerning, the next step is typically an excision or punch biopsy to remove a tissue sample for laboratory analysis.