“... The wonderful, sharp-edged Pseudomorphs of blue chalcedony from Trestia after fluorite crystals are noteworthy - and known from Romania for centuries -..."

writes Johann Zenz in his book “Agates II”, Bode Publishing House 2009

Very similar structures the thundereggs with blue chalcedony from Cannakale in Turkey are showing in their center. However, in this material the precursor mineral was trigonal calcite, not cubic fluorite. Rhomboedrical crystals with inner angles of nearly  90° are showing spoof cubic symmetry here.

The striped chalcedony from Springboek is similar in colour and banding.