18ct White Gold Blue John Rectangle 0.30 Carat Diamond Necklace. SH1_16. This stunning, sophisticated pendant boasts an impressive faceted rectangular piece of rare Blue John stone that has been sourced directly from Treak Cliff Cavern in Castleton. The stone has been claw set within 18ct white gold and crowned by three diamonds totaling 0.30 carat for an eye catching finish. The pendant hangs from a fluid 18 inch long 18ct white gold chain. All our Blue John jewellery is designed and crafted within our very own workshops in Ashbourne, Derbyshire where a team of skilled craftsmen turn the raw Blue John stone into these stunning pieces of jewellery. You can find other matching Blue John jewellery and other Blue John necklaces on our website. 18ct White Gold Blue John Rectangle 0.30 Carat Diamond Necklace. 18 inch long 18ct white gold belcher chain (42089).