Lot 391. 12.51 carats Ceylon alexandrite and diamond ring. Estimate 115,000 — 170,000 CHF. Lot sold 242,500 CHF. Photo: Sotheby's 2012

The cushion-shaped alexandrite weighing 12.51 carats, set within surrounds of brilliant-cut and baguette diamonds, size 51, images display colour change. 

Accompanied by Gübelin report no. 11030227, stating that the alexandrite, is of Ceylon origin, with no indications of heating, with a pronounced colour-change. Together with an appendix stating that this alexandrite possesses a high degree of transparency, combined with a strong colour-change effect, from brownish-green when viewed in daylight to purplish-red upon exposure to incandescent light. In addition, this gemstone has a finely proportioned cut. The combination of high clarity and pronounced colour-change effect in an alexandrite of this size is rare.

Sotheby's. Magnificent Jewels, Geneva, 14 Nov 2012