Board Thread:Questions and Answers/@comment-45295530-20200520164244/@comment-2182964-20200520170946

The only building materials I remember that are expressly linked to the Quincy are soul-synthesized-silver and soul-synthesized-glass, which are mentioned in Chapter 214 of the manga. These materials are what Ryuken's secret training ground is composed of.

No mention of the silver being used to make the crosses that I'm aware of, though.