Jean Skinner, a beauty technician from Uckfield, East Sussex, shared the warning on Facebook after she had a client come in with a black line on her nail.
The client, who's not been named, requested a manicure with a dark colour to cover up the marking, as Jean explains in her Facebook post.
Image: Facebook/Jean Skinner
'I had a walk-in nail client a couple weeks ago. She had been going to the 'NAILS' type salons for years. She had a straight dark vertical stripe down her nail. She said as soon as she sat down- I need a color dark enough to cover this stripe,' she explained.
Other salons had looked at the woman's nail and diagnosed her with conditions like calcium deficiency and blood blister, but no one had mentioned the possibility that it could be cancer, until Jean did.
After she told her client to see a doctor immediately, the woman called her to confirm the diagnosis, which was unfortunately an aggressive type of melanoma.
'This is melanoma!!! I did not want to frighten her but I told her she needed to see her doctor immediately! She called me today to tell me that yes it was a very aggressive melanoma that has already spread to her lymph nodes!! Her prognosis is not good!'
Jean continued to urge everyone to carefully check their nail beds and see a doctor if they notice any unusual changes.
'Please pay attention to abnormalities in your nail beds!! Odd changes in your nails can very likely be nothing to worry about! But sometimes it is an indication of a very serious disease', she wrote.
'And please keep an eye on the nail beds---toes and fingers----of your elderly loved ones and your loved ones that aren't physically able to notice changes in the nail beds! Early diagnosis can make all the difference in the world!!!
In the comments section of the post, where some users said they never heard of this being a symptom of cancer, Jean also revealed she once diagnosed another patient's liver cancer.
'I 'diagnosed' cancer of the liver treating a patient's foot once', she wrote.
The NHS advises that you see your GP if you notice any changes in the colour, texture, shape or thickness of your nails and you don't know why. They also say dark stripes shouldn't be ignored as it could be a symptom of subungual melanoma.