Pseudo-Octahedral Ruby Crystals from Jegdalek, Afghanistan
Contributed by: Michael Crawford
Date: Jul 29th, 2025
Locality: Jegdalek ruby deposit, Surobi District, Kabul, Afghanistan (See on Mindat)
Size: 4.5 x 4.5 cm
Description:
Corundum var. ruby (Al2O3) crystals in a marble matrix from Jegdalek Ruby Deposit, Surobi District, Kabul, Afghanistan. Ruby crystals commonly have a hexagonal habit, and this habit also occurs at Jegdalek. Ruby crystals from Jegdalek frequently have a second crystal habit, pseudo-octahedral. These ruby crystals look like spinel (MgAl2O4) crystals. Ruby and spinel have bright red fluorescence under longwave UV illumination and their fluorescence is dimmer under midwave and shortwave UV light. These pseudo-octahedral ruby crystals from Jegdalek are sometimes identified as spinel because of the bright red fluorescence and crystal habit. There are listings in mindat that describe spinel from Jegdalek that should be re-evaluated (https://www.mindat.org/photo-3269.html).
The longwave emission spectra of ruby and spinel are very different, and measurement of their emission spectrum is a fast and non-destructive method to discriminate ruby from spinel. The ruby longwave emission spectrum has a very bright peak at 692 nm. This peak is nearly 4 to five times brighter than peaks (659 nm, 669 nm, 706 nm and 713 nm) on either side of this prominent peak. No peaks in the spinel longwave emission dominate its signature. The spinel spectrum has five peaks (673 nm, 684 nm, 696 nm, 706 nm and 715 nm) that makes it distinct from the ruby spectrum.
The longwave emission spectrum for this Jegdalek specimen matches the ruby spectrum. Other pseudo-octahedral crystals from Jegdalek in my collection are all rubies base on their longwave emission signatures.
Summary of luminescence responses:
Corundum var. Ruby (Mindat) (RRUFF)
- Fluorescence under Longwave (365nm LED) UV light: Red