Could the Crystal Masons be tied to/perhaps run by the Tiefling merchant dynasty mentioned in Rain's background?
1.) It'll save me re-writing too much.
2.) As Tieflings, they may be regarded as "not quite one of us" by the Kingdom of Jade's successive rulers anyway, even if they have lived there since *a long time ago.* Within the Kingdom of Jade, they may have filled an Untouchable role that doesn't fit within the courtly caste system (not unlike certain real life minorities in this world).
1.) It'll save me re-writing too much.

2.) As Tieflings, they may be regarded as "not quite one of us" by the Kingdom of Jade's successive rulers anyway, even if they have lived there since *a long time ago.* Within the Kingdom of Jade, they may have filled an Untouchable role that doesn't fit within the courtly caste system (not unlike certain real life minorities in this world).