Will, I used to travel internationally on business quite a bit, and
was always advised when carrying anything of value that I should have
ample documentation for that item with me. Particularly with jewelry
and similar items, I was told to carry the receipts for the items,
showing when and where it was purchased. Otherwise, it's up to the
inspector at customs to decide whether s/he believes you about its
point of origin.
In today's world, quantities of hard currency, precious metals, and
gems are used in money laundering schemes, so it's especially
critical that you be complete in your documentation to avoid
suspicion. Also, you're traveling to an area where people go to BUY
these things -- so it would be a possible red flag on re-entry (or on
leaving Belgium) that you might be trying to avoid significant duties
on purchases made there. Having your U.S. receipts handy (copies of
them are encouraged, not originals) could save a bit of a hassle.
Just a thought!