color-change rutile???

Is rutile capable of changing color from red/orange to green under different lighting conditions? I'm in the middle of moving back to my home state (Yay!), so will be checking back in a bit, so thank you in advance!

Up to my knowledge; no.
Greetings
Guy Kronz

Not as far as I know..... But it is (often) pleochroic, brownish red/yellow/green. In some stones a strong pleochroic effect is seen even without a polarizing filter. Try checking for this before leaping to conclusions. Don't forget to rotate the stone in three and not just two dimensions.

Are you talking about the various colors of the mineral itself or are you talking about color change in transmitted light?

Rock Currier
Crystals not pistols.

Big topic... For crystals other than the cubic system, a singe light ray entering the crystal splits and then takes two or three different paths through the crystal. If the stone is also coloured, very often the light in the differently polarised paths shows a different colour, though the illuminating white light source in no way changes. This effect is not always present and, if present can be observed weakly, of medium strength or even strongly. This effect is called pleochroism and where detectable can help identify the crystal. To detect the effect, a simple light polarising device, the dichroscope, is commonly used as quite subtle colour changes with the light polarisation can be observed with it.

However, in some crystals, the pleochroic effect effect is so strong as to be clearly observable with the naked eye just by placing the (transparent) crystal over a white light source and rotating it on all three axes, whilst watching for a change in the observed colour.

Here's a simple example. The crystal is a piece of Sapphire gem rough (probably Australian) that is strongly pleochroic, showing as blue or green depending orientation of the stone. The stone is placed on a diffusing surface above a white LED. and photographed from directly overhead.



The crystal fragment is rotated through 180 deg horizontally as it lies on the light table; no change in the colour of the stone is observed at any point on the rotation. The colour is uniformly blue.



The stone is then photographed with the stone turned through 90 deg (onto one side) and rotated horizontally about that axis. The stone now transmits the light from the same whit LED as a good saturated green.



Sapphire will, fairly reliably, show two-coloured pleochroism and such is sometimes called dichroic. Not only is this effect helpful in ID-ing an unknown crystal but it can be of great importance to an expert cutter to orient the cutting so that the desired colour will be the one seen when the cut stone is mounted in a piece of jewellery. It the cuts are not oriented according to the stone's pleochroic colouration, the results can seem strange.

In sum, the quality of the illuminating light here is unchanging. The colour of that light when transmitted through the crystal can depend on the orientation of the crystal to the light source and an observer. 'Gems' lists in an Annex about a hundred anisotropic coloured varieties of gemstones with their typical pleochroic colours, which is a useful general guide. Two species that commonly demonstrate strong pleochroism are Andalusite and Tanzanite.

The term 'colour change' should be reserved for a different effect, that where a crystal transmits a different colour not according to its anisotropic nature and orientation but according to change in the composition of the spectrum of radiation that illuminates it.

It is possible for both effects to be present in one and the same stone. they can be distinguished and the separate effects of each noted by running two sets of tests, one for pleochroism (by changing a crystal's orientation) and the second for 'colour change' (by changing the type of illumination in which the crystal is viewed).



Edited 1 time(s). Last edit at 08/13/2012 08:00PM by Owen Lewis (2).