For any of you who think on viewing this post you are suffering deja vu, worry not. It replaces a post with the same details made some three weeks ago and which mysteriously vanished.
The transmitted lighting for the attached images was linear polarised but not cross-polarised. The specimen is a 7ct blue spinel (kindly loaned by friends for the photography), showing multiple inclusions of different mineral species, mainly anisotropic unlike the host crystal.
Thank you for these amazing pictures and for all your efforts when the forum software was misbehaving.
We are running an update to the forum on our development server and we have to update the 'skin' now too.
Whereas 'false colouration' caused as a thin film effect can be found in both isotropic as well as anisotropic crystals, thin film interference effect does not need a polarised light source to be seen. Anisotropic crystals alone can show such colour when illuminated with linearly polarised light. However, a third cause of such colour, occurring in transparent, amorphous and crystalline materials) a a result of rotational polarisation. In this, the cause of the colour is physical strain set up between the host material and foreign bodies included within it. Perhaps most gemmologists will never see this effect except in copal and amber, in which its occurence is a commonplace occurrence for all who use linearly polarised light to illuminate their transparent amber and copal specimens.
As we know, the older the polymerised hydrocarbons become (with all else being equal), the more stiff and brittle the material (by now old amber) becomes.
As this stiffening begins, rapidly (sometimes within months) physical stresses begin to set up in the material at boundaries to some different material enclosed within it. When the material is illuminated by linearly polarised light, the plane of polarisation becomes rotated to some degree. There then can be interference between light in the original illuminating polarised plane and the rotated plane and that interference causes the appearance of colour.
The first signs of this are found in copal where black and white 'cross' patterns, of light strengthening and extinction, are set up around small inclusions. These are reminiscent of the axially related isogyre interference patterns that can be set up in uniaxial crystals. With favourable environmental conditions and the passage of millions of years, copal changes further, stiffening a point where we call it amber - and with internal stresses increasing as a result of further chemical change - principally, the loss of volatile organic compounds through evaporation. The rest of this post concerns itself only with polarised light effects in copal. The change in polarisation effects in amber, 30 - 100+ million years old, will be covered in a following post or two.
Image 1. A scene in Madagascan copal illuminated with transmitted, linearly polarised and ambient room lighting. The specimen (as for all my copal/amber images in this topic) was immersed in water to remove any exterior surface effects. The spider is approximately, 1mm nose to tail
Image 2. A detail from the same specimen showing the interference effect around what look like grains of pollen.
From its associated geology, Chiapas (Mexico) amber is said to be about 22-27 million years old, which is still pretty young for amber. After its attractive appearance, perhaps the key qualities for amber to be desirable as a raw material for the artisan's bench are (1) hardness (loss of plasticity) and yet to be (2) not too brittle. However, it is common for Chiapas, along with other types of amber, to have strain locked into it, increasing a tendency to crack. This strain can be found both between the amber host material and inclusions within it and also within the amber alone. Strain is set up within the amber alone by either or both of the following:
- Copal (and hence amber) is usually the result of repeated flows of resin at different times of origination from the host plant, with environmental conditions and ageing combining to cause strain within amber at the boundaries of different resin flows.
- As light and oxygen affect the resin's hydrocarbon molecules, the molecules polymerise. I.e. the resin molecules join together to form very long chains with repeating chemistry blocks.Other chemicals present in the resin form short lateral bonds between adjacent polymerised molecules, serving to bind them together. At a speed influenced by environmental conditions and the passage of time (usually eons in the case of amber), strain can form between adjacent polymer strings as the chemicals bonding them together are slowly leached out of the amber, eventually causing the remaining bonds to be over-stressed and fail, with the formerly solid amber 'coming apart at the seams'.
The shows of colour that can be produced in some amber when viewed in transmitted and linear polarised light give us an excellent guide to the risk of the amber breaking in the course of being worked or even coming apart seemingly spontaneously at some time after the working has been completed.
The more light interference colours that can be seen and the closeness of the colours together give a clear indication of the level of stress locked into the material and wanting release. For around a hundred years, engineers have been building scale models of structures out of transparent plastic ,to determine by viewing the models in polarised light, where there are any over-stressed parts that must be re-designed before the real construction begins..
Image 1. This shows the same interference pattern formed around a solid inclusion as was shown in copal in the previous post. However, in this Chiapas amber the stress has increased enough for colour to begin to show.
Image 2. A stronger show of colour around what seem to be liquid filled (and hence incompressible) bubbles in the same piece of amber as image 1. It can be seen that the very smallest inclusions cause virtually no stress to be locked into the amber surrounding them.
Image 3. This shows another piece of Chiapas amber with no sign of stress between the amber and the pieces of plant material included within it. However, there are coloured bands, fairly close together as a result of stress being formed for some reason between the amber polymer molecules themselves.
Image 4. An enlarged detail from Image 3 with the bands of interference colouration showing closer to their true vividness by balancing the image to remove colour masking from the fairly deep red-yellow body colour of this Chiapas amber itself.
Many of us have seen it written somewhere or other and from time to time that the presence or absence of light interference effects caused by passing plane polarised light through some resinite specimen is a method of differentiating copal from amber. Any curious enough to check this 'factoid' for themselves, over maybe a hundred or so mixed samples of both, are likely to consign this idea to the bin of 'urban myth'. It just ain't so, as can be shown.
There are indications to this effect, in the few copal and amber images displayed in the last couple of postings above. Both constructive and destructive interference patterns (white and black) are clearly shown in Madagascan copal viewed with a plane polarised light source. There is no amber found in Madagascar and my Madagascan material will dissolve over time in acetone. In Mexican amber, light interference patterns and colours can be found, both caused by physical strain between the host material and inclusions in it and between layering in the amber itself.
But are patterns and colours from light interference always to found in amber? It seems not. Examining Baltic amber specimens gathered from several different Baltic sources, showed that:
1. Frequently, no strain effect can be detected when viewed with polarised light (see images 1 & 2 below) .
2. In all my tests, when strain could be detected in the body of the material it was always seen as black and white. In a three-figure number of trials, I have not yet seen discrete colours resulting from light interference to be caused in Baltic amber.
Image 1. Baltic amber with inclusions. Strain interference? Possibly, in just one or two places, but so weak as to be easily overlooked.
Image 2. Same piece as Image 1 but refocussed to make sharp the 'leggy' inclusion, just below the spider's abdomen. This inclusion is in fact, a plant material commonly referred to as 'oak bud hair' and often found in Baltic amber - though not in amber from elsewhere (Life in Amber, p 77-78), Poinar G.O.,Standford University Press 1995 reprinting. No sign of strain between this inclusion and the host amber.
Image 3. The death throes of what is possibly a fungus gnat (family Sciaridae). Any strain interference present is vestigal. Compare with the images of Madagascan copal and Mexican amber.
Since Mexican amber is only roughly half the age Balltic amber (the latter typically 40-60 million years old), could it be that this huge amount of extra ageing causes a reduction in strain interference? Flatly, no. Burmite (amber from Burma (now Myanmar) is half as old again as Baltic amber and yet strain interference in burmite can be vividly varied in colour when viewed with plane polarised light (indicating a high level of stress present).
In sum, it can be shown through patient testing with simple optical equipment that copal and amber cannot be reliably (or even indicatively) differentiated on the basis of of light interference caused by strain. To this observer, at least, the signs are that the age of the material is inconsequential in detemining what one sees. Rather the key influences are:
1. The chemistry of the original resin (not always neatly differentiated according to the modern situation of deposits).
2. Environmental conditions in which the material has been stored throughout its life. A matter for informed speculation and not for strict proofs?
Anyone else have any thoughts on this matter? Food enough for thoughts on this to fill several gemmo-oriented threads, methinks.
Possibly the most basic thing a gemmologist wishing to pick up photomicrography (or macrophotography) needs to understand that our photography has some special requirements. Photography for most other people is about capturing daylight (or room lighting that has been reflected off of the nearest-to-camera surfaces of objects to produce an image of those objects. However, for most of gemmology-related photography this is not so:
- We need to select our light source to best suit our needs and illuminate for us information that is useful to us and frequently wondrous.
- Most often we do not want light to be reflected back to our camera from the nearest surfaces but, before we make an image with captured light, we first want that light to have travelled through the crystal or other substance being photographed, interacting with either the substance itself or with some different material(s) that are included within the crystal etc.
So a gemmologist learns to select and use his light sources according to the particular substance and investigations he is making at the time. Carefree, happy snapping might seem like fun, but for the gemmologist, it rarely brings satisfaction. IMHO, that comes from learning how to use light sources like a golfer uses the different clubs in his bag; a putter for those accurate and gentle final pushes and a driver for smacking the ball a couple of hundred metres down the fairway. Select the wrong club and the result is rarely satisfying.
Another way to get the best images is practice, improving one's images by manipulating the image data in a computer after the image has first been captured in a camera. Done skillfully, there can be real improvements to image quality by using a good software programme, such as Adobe Elements. Emphatically, this is in no way cheating (except for faking an image). From the very first days of photography, skilled photography has relied at least as much on post-camera image processing as on the first formation of an image in the camera. In these days of digital cameras and powerful computers of everyone's desks, this is now truer than it has ever been
All that said. there are times when the photographer really does need his light to reflect off the subject's surfaces closest to the camera. Usually, this will be where a critical examination of some surface feature is required. Some of these features may have a very shallow depth (a few microns only). Such features can be invisible to the naked eye and in daylight, it requiring a light beam to illuminate the surface with an angle of incidence that makes it almost parallel to the surface being examine.
Image 1. Here's one example of a shallow feature in kunzite on a cleavage surface. Why in the longest side in each of the 'triangles' curved and what is the significance of that? Any suggestions?
Image 2. Another interesting kunzite image, capturing spodumene's signature perfect cleavage in two directions with an uneven, splintery fracture. No wonder that cutters find spodumene so tricky to work with!
Inage 3. More 'triangles'. This time growth features on a nearly flat fracture surface on a chip of morion. In quartz, these growth features are close to isoscolesian triangles in 2-D form but with equal curvature in each of the two longer sides. And the cause is? What does the team think?