The Crystal Skulls So Far… 
The crystal skulls first appeared in the Far West saga, where it’s hinted that around the 1850s, some crystal skulls are found/smuggled by Aldous McGaughran. Their origin is not told.
It seems that (at least) one of his crystal skulls are passed down to Claudio in Spain through his grand-father’s acquaintance of Cillian Mc Gaughran (one of Wrick’s ancestors) — ref.
That skull is auctioned and a lady in salmon (the fake viscountess who is in reality an agent of the Mad Baron) gets it. This skull finishes its trip in the Baron’s lair (at around our time ~2007)… The Baron’s mansion will become (in the 2030s?) the home of the twins, and Wrick family.
Some of the crystal skulls are also found in the past (1950s?) around the mysterious figure of Mrs Chesterhope who is already hunting for them in (Brunei?) sultanate, using Georges to do so.
Later (around 2008) she locates one on the island of Tikfijikoo, and she sends a gang of magpies to find them, but their efforts are thwarted and she needs to get there in person (and motorcycle).
The Confregation is an organization which seems to know some things about them and are the origin of the one lent to the Dr Bronkelhampton on Tikfijikoo (retrieved from Crusaders a long time ago).
Beattie and Leonora Fletcher, a couple of batty Brit ladies seems to have found some of them too , and have a network of their own…
Later (2030s?), near the Indian Ocean, one is found by Gayesh’s family too …