@nene78 I'm about as far from an expert as possible in chemistry but a simple lookup will tell you that strontium is a metal. You may want to get it from algae which has absorbed it rather than from a chunk of rock but it is still a metal.
From Wikipedia: " An alkaline earth metal ... Strontium is a divalent silvery metal with a pale yellow tint whose properties are mostly intermediate between and similar to those of its group neighbors calcium and barium.[10] It is softer than calcium and harder than barium. "

To me the important points are:
How well does it work to prevent fractures?
If it increases bone density is it building stronger and functional bone?
What are the possible side effects?
Are the problems with strontium ranelate likely to occur with strontium citrate?
Are the studies of strontium ranelate directly relatable to strontium citrate?

I admit I haven't investigated strontium enough and I do intend to investigate more.
I do know that the logic that strontium citrate is natural and meds are unnatural is not a sound basis for thinking it's ok to take. Large doses of trace minerals can be quite dangerous no matter how "natural" they are. The 640mg dose of strontium citrate is way way greater than the 1-5mg we would likely get per day in food.
Still, it would be great if strontium citrate was really an effective agent for bone thinning.