Experience with loose stone detector

(it’s that stand with a magnifying glass and the little thing that
vibrates so you can see all the loose stones vibrate in whatever you
hold against it.) seems like it may be too good to be true. but if
customers could see their own loose stones, it would surely increase
business for tightening stones. just curious if anyone has tried one
of these… does it actually do what it’s supposed to?

thanks,
dax