Ruby and Kyanite from Australia
Contributed by: Michael Crawford
Date: Jun 10th, 2025
Locality: Harts Range (Harts Ranges; Hartz Range; Hartz Ranges), Central Desert Region, Northern Territory, Australia (See on Mindat)
Size: 7 x 12 cm
Description:
This is a specimen of teal kyanite crystals and red corundum var. ruby in a matrix of black biotite. The specimen is from the Harts Range, Northern Territory, Australia. The kyanite and ruby fluoresce red in LW illumination. There is no fluorescence in MW or SW illumination.
Longwave emission spectra show sharp peaks in both ruby and kyanite. Ruby has a very intense peak in the visible region at 692 nm and much smaller peaks at 679 nm and 713 nm. Kyanite has two sharp peaks at 688 nm and 705 nm. These sharp are caused by chromium (Cr3+) ions replacing aluminum (Al3+) in ruby and kyanite. Kyanite fluorescence is the most intense in the near infrared which is invisible to our eyes. The kyanite spectrum also has a broad peak with a maximum at 755 nm in the near infrared that is also activated by chromium but replacing a different aluminum site in the kyanite crystal lattice.
The last two images show the near infrared fluorescence of the ruby and kyanite. Kodak used to make a color infrared film that imaged green and red visible light and near infrared light that could be used with any 35mm film camera. When digital cameras replaced film cameras, the ability to take pictures of near infrared was lost. Digital cameras can be modified to capture infrared images with special external filters on the lens. The first image recreates an image that would be taken with color infrared film. It is composed of images of green visible light, red visible light, and near infrared light assigned to blue, green, and red respectively. Ruby is green and kyanite is yellow in this image.
The second infrared image is a composite of three narrow-band, near infrared images centered at 720 nm, 770 nm, and 860 nm. The locations of the bandpass filters are shown on the emission spectral plot. The 720 nm image captures fluorescence from ruby and kyanite. Whereas the 770 nm and 860 nm images only capture fluorescence from the kyanite. The resulting false color image shows ruby as orange-red and the kyanite is white.
Summary of luminescence responses:
Corundum var. Ruby
- Fluorescence under Longwave (365nm LED) UV light: Red
- Fluorescence under Longwave (365nm LED) UV light: Red