Calcium ions are abundantly found in the extracellular space of the stratum corneum area, the top most layer of the skin (Skin Barrier and Calcium; https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5929942/). Isn't it very probable that it can also be found on the surface? Calcium ions are also actively involved in regulating the water gradient of the stratum cornuem. However, they may be easily washed off from the surface of the skin through the regular use of water and surfactants. Plus, the use or over-use of moisturizers also interfere with natural sebum production and the Calcium ion gradient of the stratum corneum ( The Changes of Epidermal Calcium Gradient and Transitional Cells after Prolonged Occlusion Following Tape Stripping in the Murine Epidermis; https://www.sciencedirect.com/science/article/pii/S0022202X15405603) If calcium is found on the surface of the skin, and sebum plus intercelular lipids (which also eventually make their way to the surface of the skin and mix with sebum and sweat to form the acid mantle) contain cholesterol, wouldn't it be possible then Vitamin D could be synthesised on the skin through irradiation?