In this battle between pretty blue but very rare gemstones, I am voting Jeremejevite, which is pleochroic with colour changing with viewing angle and piezoelectric, over Haüyne, formed in magma chambers, even though it can fluoresce.
It’s a tough call. Art & physics arguments for both.
Rare Jeremevite and Alexandrite Ring
lakaka, Madagascar
via https://www.bonhams.com/auction/26600/lot/3037/rare-jeremevite-and-alexandrite-ring/
Ring in platinum with 2.48 ct. cushion cobalt blue spinel, 0.06 ct. t.w. hauyne, 0.28 ct. t.w. Paraiba tourmaline, and 0.22 ct. t.w. diamonds,
Via https://www.jckonline.com/editorial-article/omi-prive-hauyne/