That's because those aren't Polish crown jewels. But there's an entire story to that.

In late 18th century Poland was invaded and divided by Russia, Prussia and Austria. Polish crown jewels were captured by Prussians and, in an act of incredible barbarism, melted down. The only part of Polish crown insignia that survived in the sword named Szczerbiec, currently kept in the Wawel castle. Szczerbiec itself several times barely escaped destruction.

Fast forward to 2000s. A professional association of Polish jewellers recreated the destroyed Polish crown jewels. They used Prussian gold coins, nodding that at least some of the gold used might have been part of the original jewels. One such Prussian coin is displayed.