Scales

The Mark is a manifestation of magical potential, taking the form of a silicon gem typically in the forehead and forming during puberty. This gem is integrated into the person's skull (or nearby bone in the rare cases it manifests elsewhere), and certain silicate compounds are often added to meals by hopeful parents or knowing magi. This potential doesn't always visibly manifest however, and sages have discovered over the years that silicon pathways exist in most beings to some extent. While this indicates magical potential, it also leads to problems in old age, as the silicates begin manifesting as scales.
These scales can be uncomfortable or even painful, and depending on where they grow they're quite an inconvenience and require treatment. Many elderly manifesting scales leave most of them alone, as a few scales on the back or belly are of little significance. Many grow into beautiful gems, though nearly everyone considers using human scales for jewelry to be taboo. The only exception is elderly wearing their own removed scales as jewelry, which is quite popular in some cultures as a sign of age and wisdom. Decorative patterns woven from a deceased elder's scales often adorn graves or the sacred trees planted above remains, and in one region, affixing scales to ever expanding family mandalas is customary, creating a beautiful tapestry showing family history.

Causes

Old age and magic aptitude factor into who manifests scales. Any human has the potential, but it's generally assumed that the more potential one has the greater the chance. Magi are somewhat famous for growing considerable patches of beautiful scales.

Treatment

Salves and oils are used to moisturize afflicted areas and attempt to make people more comfortable. In some cases, the scales are removed outright, which is itself a painful process but often times necessary.