Preliminary results indicate no shells, couldn't find any in a tablespoon rinsed sample but lots of black and dark grey stone which does not look like quartz. I suspected limestone.
Added vinegar. Lots of bubbles, more than with my shell sand but it's been a while since that test so that's hardly an empirical measure... although I was surprised at that.
Sorting through the grains to determine exactly which ones were bubbling revealed that the black and dark grey stone pieces are the culprits, limestone. Seeing as limestone is mostly some form of calcite which is primarily crushed skeletons of various sea creatures, probably anything from corals and whatnot.... it might as well be shells, just a heck of a lot older.
Recall the 99% to 99.9% pure silica quartz by weight? I would be surprised if it was as high as even 99%. I left the sand and vinegar in a bowl to evaporate overnight, basically the same as I did testing the sand that I used.
Pic one is my sand.(source uncertain, Home Hardware supply)
Pic two is the Quickrete Play Sand.
I was surprised yet again as there was far more deposited than I anticipated. While I am not trying to prove one sand source is better, I thought it prudent to point out that both are similar with regards to there being calcium content. If I were trying to prove anything beyond this, I would make certain that the comparison was more empirical and include other sources while I was at it.
So if you really really need to eliminate any form of calcium source from your aquarium I guess your best bet is to avoid playsand. I might do a test with water sometime and see how much, if any, calcium is deposited.
Seeing as many here have used it for many years with no problems I see no reason not to use it for the vast majority of tanks. I actually see the KH remain constant or drop very quickly after water changes, I have not figured out the process behind the drop yet but obviously the calcium in my sand seems to have little bearing on this.