Alexandrite from Tanzania
Contributed by: Michael Crawford
Date: Jul 29th, 2025
Locality: Lake Manyara, Arusha Region, Tanzania (See on Mindat)
Size: 11 x 14 mm
Description:
A small crystal of alexandrite (BeAl2O4) from Lake Manyara, Arusha Region, Tanzania. Alexandrite is a chromium bearing variety of chrysoberyl that is green in sunlight and changes to purple under incandescent light. Broad spectrum sunlight and some LED’s that are rich in blue and green wavelengths are preferentially reflected by alexandrite making it appear green. Incandescent lights are richer in red wavelengths and alexandrite reflects more of the red wavelengths, giving it a purple color. This type of color change is observed in other minerals (i.e. garnet, fluorite, corundum, etc). This property is referred to as the “alexandrite effect”.
The chromium in alexandrite activates red fluorescence under longwave UV light. Chromium (Cr3+) replaces aluminum (Al3+). The emission spectrum has a sharp peak at 679 nm and a slightly wider peak at 702 nm.
Summary of luminescence responses:
Alexandrite (Mindat) (RRUFF)
- Fluorescence under Longwave (365nm LED) UV light: Red