Iron is the only magnetic metal i know of , maybe ruhbidium is magnetic but if thats the case you can afford to take it out sell it and fill it with a layer of solid gold.

It is iron that makes it magnetic, although this is not to say it is the only metal in the sand. The company declares the sand 'reef safe'. The comments from those urging you to remove are likely based on the 'better safe than sorry' approach, although I am just guessing. You can always contact the manufacturer and ask them :)

The magnetic parts of your sand may wreak havoc on your power heads and pumps ... both their impellers and the impeller housings

To be safe I would put some sand in a small volume of water for a while and test that for heavy metals like copper.