Board Thread:Questions and Answers/@comment-45295530-20200520164244/@comment-5551986-20200520214351

Beats me, I agree with Kenji however, and, as an assumption seeing as Quincies reference silver sometimes, I'd hazard a guess as they are made of silver judging by the colour and obviously molded into the form of a cross.

Mythically its always been revered as having "magical" properties. Ryuken built a place out Soul-Made Silver, and they get silver in their heart after Yhwach does his "bizniz".

In terms of how it works, it probably just naturally draws Reishi to it.