Hi Moonbeam
There is an excellent book by Mark Sircus - transdermal magnesium.
I am not sure why you ask the question so specifically, however, that book (and I am sure others) covers the plethora of health issues affected by low magnesium and the importance of having enough in your system.
Because of the problems in tolerating some forms of magnesium, this is a very easy way to supplement. You can use it in your bath or a foot bath, spray on your body as you require or regularly.
I use the BetterYou range of magnesium Spray - normal and night time and use on and off.
I also buy quality magnesium flakes for the bath and just have that as and when I feel like it.
I, too, had fibromyalgia and found that it helped (once I discovered it!).
You can actually make your own magnesium oil spray by using magnesium flakes and mineral water in a spray bottle.
I really would recommend that you read about it first and decide what suits you.
Fx