Duncan Miller

There are several different mechanisms for garnets to show a change of colour. The most common of these is analogous to the colour change in the well-known alexandrite variety of chrysoberyl. This is due to differential transmission of different wavelengths of visible light, leading to a difference in perceived colour under lighting with different degrees of red or blue light. A less well-known type of colour change in gemstones is the so-called Usambara effect, named after the East African locality that produced the chromium-bearing tourmalines in which this effect was first recognised (http://www.nordskip.com/usambara.html).

This colour change depends on the thickness of the material. A single green stone may look red when another one is placed on top of it. This is illustrated and described in an online SSEF publication by Dr M.S. Krzemnicki, first published in Facette 21 (February 2014) https://www.ssef.ch/exceptional-colour-change-garnets-showing-the-usambara-effect/. In larger single gemstones displaying the Usambara effect, the difference in thickness between the centre and the edge can produce different colours – red in the centre and green on the edges. This is also illustrated in Dr Krzemnicki’s article. “This specific effect, first noted by Manson & Stockton (1984) as “colour-shift” on colour-changing garnets was only later fully described by Halvorsen & Jensen (1997) on Cr-bearing tourmaline from the Usambara mountain range, and thus named the Usambara effect.  It describes the property of a material to change colour in relation to the path length that light travels through the material. In the case of our garnets of equally greenish colour in daylight, we can observe a dramatic change of their transmission colour into red when held together and illuminated with a strong light source from behind (Figure 3). This specific “colour change” is not related to different white light sources (as is the alexandrite effect, i.e. classical colour change), but is directly linked to the different path lengths that light travels through the material when viewed alone or when put together (resulting in a “doubling” of the path length).

Further reading:

This link contains more information and photographs of this intriguing colour-change effect. https://www.gemsociety.org/article/what-is-the-usambara-effect-in-gemstones/

Duncan’s Garnet Group chart