[michaelt]

Grocery store cashiers and bank clerks often don't get to leave until they've accounted for every cent that should be in their till.

One would think diamond merchants would take the time to be equally careful, if not moreso.

[BobAliceInATree]

Most of these businesses in the district are tiny, family-owned businesses, so there's a huge amount of implicit trust there.

But also, these tiny stones really aren't worth much. What you're asking is like Home Depot making sure to account for every single nut & bolt so none are stolen or lost. It would cost far more in time & labor then what you'd get back in return.