Hydrated magnesium chloride can be extracted from brine or sea water. In North America, magnesium chloride is produced primarily from Great Salt Lake brine.
There are many people on the net praising the miracles of magnesium including pain relief, heart health, relaxation, improved brain function and healthier skin, that I tend to think using magnesium as an ingredient is probably okay. In fact, a few sites say that skin application it is a great way to absorb Magnesium for nutritional value.
In fact, I believe the principle of soaking in Epsom Salts for aches and pains in the bathtub is a means to deliver Magnesium Sulfate to the body through the skin, relieving magnesium deficiency and hence, aching muscles.