Journal Digest: Gem-Quality Amethyst from Rwanda, Optical and Microscopic Properties

Guy Lalous ACAM EG explores gem-quality Rwandan amethyst in his latest Journal Digest, from the Spring 2018 issue of The Journal of Gemmology Volume 36, No. 1.

This article describes the optical and microscopic aspects of Rwandan amethyst, mainly those reportedly from the Ngororero District.  Two areas have been cited as potential sources for the new material: the Ngororero and Ruhango Districts.


Figure 1: Amethyst crystals from Rwanda display six rhombohedral faces, occasionally in combination with short prism faces. The rhombohedral r and z faces are highly distorted, with symmetry-equivalent faces differing in size. The sample weighs 68 g and measures 50 × 45 mm. 
Photo by K. Schmetzer.

XRD Testing

X-ray diffraction (XRD), as noted in the previous Journal Digest, is a method to identify minerals and crystalline materials.  The method is based on the scattering of X-rays by the crystals. 'X-rays are diffracted by each mineral differently, depending on what atoms make up the crystal lattice and how these atoms are arranged... An X-ray scan provides a unique "fingerprint" of the mineral' (M.S. Pandian 2014).

The morphology of amethyst crystals from Rwanda comprises primarily rhombohedral r and z faces, occasionally in combination with prismatic m faces (as noted in Figure 1). Most of the Rwandan amethyst crystals are distorted and broken.  XRD of the encrustations on the crystals identified them as opal-CT (whitish coatings) and lithiophorite (black coatings).


Figure 2. Morphology of amethyst crystals. Shown here are (A) an idealized clinographic projectionand
(B–D) idealized (non-distorted) projections viewed parallel to the c-axis showing different sizes of r and z faces. Drawing by K. Schmetzer. 

What can Pleochroism tell us about Rwandan Amethyst?
Pleochroism is the appearance of different colours in different vibration directions through an optically anisotropic material, due to the differential selective absorption in the two plane polarised light rays vibrating at 90° to one another.  In a uniaxial material, two different colours may be seen, which is termed as dichroism, and a biaxial material may show three colours, leading to the term trichroism.  Pleochroism is never seen in an isotropic material, as the light waves entering the material are not reorganised by the crystallographic structure of the material in question.

On testing the samples of Rwandan amethyst, this article demonstrated that that some amethyst— despite being α-quartz and part to the trigonal system—shows pleochroism when viewed parallel to the optic axis in slices cut perpendicular to the c-axis (see too Pancharatnam, 1954; Lietz and Münchberg, 1956). In such slices, the three symmetry-equivalent r or z growth sectors exhibit identical pleochroism and colour. Consequently, when a sample is viewed in plane-polarised light and rotated about the c-axis, each rotation through 120° will reveal an identical growth sector with identical pleochroism. Some of the present samples from the Ngororero District exhibited this property, which was especially evident in slices cut from the crystal.

 Figure 3: This schematic representation of the pleochroism in amethyst demonstrates symmetry-equivalent rhombohedral growth sectors (violet and light purple) 
The labels X1, X2, Y1 and Y2 represent the colours observed within the different growth sectors using one polarizer between the sample and the observer. Drawing by K. Schmetzer.

Three different types of pleochroism were seen in the triangular growth sectors, depending on their colouration. In general, a particular specimen showed two of the three types of growth sectors/ pleochroism—that is, a combination of type I and type III growth sectors or a combination of type I and type II growth sectors. However, in some samples all three types of growth sectors were found. Nevertheless, because most samples featured a high degree of distortion, there were frequently fewer than the ideal six rhombohedral growth sectors.

TYPE

Sector Colour

Pleochroism

I

Light violet to almost colourless

Light violet and colourless

III

Purplish violet

Intense purplish violet and yellow to light orange-yellow

III

Intense violet

Intense violet and lighter slightly purplish violet

Table I: Pleochroism typical of Rwandan amethyst, viewed parallel to the c-axis


 
Figure 4: Viewed parallel to the c-axis,  sectorial colour zoning with two light violet to almost colourless growth sectors (type I)
and two intense violet growth sectors (type III). Photomicrograph by K. Schmetzer, in immersion; field of view 12.8 × 10.0 mm.

 
Figure 5: With plane-polarised light, in a view parallel to the c-axis, the rhombohedral growth sectors of exhibit different combinations of colour and pleochroism:
(Left. A) a combination of light violet to almost colourless (type I) and
intense violet (type III) sectors,
(Right. B) a combination of light violet to almost colourless (type I) and purplish violet (type II)
sectors.
Photomicrographs by K. Schmetzer, in immersion; field of view 14.5 × 10.9 mm.
  

Brazil-law (Optical) Twinning
Commenting on Schmetzer’s earlier work, Crowningshield, Hurlbut and Fryer’s Gems&Gemology 1986 article noted that the growth of amethyst had been correctly interpreted by Sir David Brewster in 1821 as due to the polysynthetic twinning of right- and left-hand quartz.

 
Figure 6: (A) Viewed parallel to the c-axis with crossed polarisers, this amethyst displays Brewster fringes due to Brazil-law twinning.
(B) The schematic diagram shows a crystal in the same view, highlighting the area in photo A.
Photomicrograph by K. Schmetzer, in immersion; field of view 14.5 ×10.9 mm. Drawing by K. Schmetzer.

In the present 2018 article, Schmetzer noted again that natural amethyst normally shows colour zoning with somewhat more intense violet r growth sectors.  Viewed parallel to the c-axis with crossed polarisers, the intense violet r growth sectors typically exhibit interference patterns consisting of more-or-less regular black stripes (Brewster fringes). This pattern is a result of Brazil-law polysynthetic twinning in these sectors. No such pattern was found in the purplish violet or intense violet growth sectors of the Ngororero amethyst.

In contrast, the samples revealed three scenarios with respect to Brazil-law twinning features:

  • Pattern A: No interference patterns with Brewster fringes were seen in any of the growth sectors despite the presence of different types of rhombohedral growth zones as described above.
  • Pattern B: Single Brewster fringes confined to the boundaries between different growth sectors were observed.
  • Pattern C: In addition to Brewster fringes following the boundaries between different growth sectors as described in pattern B, polysynthetic twinning on the Brazil law was observed in the light violet to almost colourless growth zones, at times resulting in highly complex patterns.

   
Figure 7: Complex arrangements of optical twinning features. 
Views parallel to the c-axis with crossed polarizers, single black interference lines (Brewster fringes) follow the boundaries between various rhombohedral growth sectors and are accompanied by dense patterns of Brewster fringes indicating
Brazil-law polysynthetic twinning in the light violet to almost colourless sectors. Photomicrographs by K. Schmetzer, inimmersion; field of view (A) 13.5 Å~ 10.1 mm, (B) 11.9 Å~ 9.0 mm and (C) 14.0 Å~ 10.5 mm.

Striations parallel to the external rhombohedral faces of the amethyst crystals were frequently observed within the rhombohedral growth sectors.

Beetle-legs?    

“Beetle-legs” were once believed to be lepidocrocite.  They consist of red iron oxide inclusions attributed to platelets or fibres of hematite in platy and needle form. 

All of the rough and faceted samples displayed colour zoning that corresponded to the different rhombohedral growth sectors. The inclusion scenery included: partially healed fractures, negative crystals, two-phase inclusions (liquid/gas) and reddish-brown solids resembling beetle-legs. 

Four samples reportedly from Western Rwanda were different as they were composed of two adjacent violet growth zones, both of which were twinned. 

Conclusions

If a faceted Rwandan amethyst consists of several growth sectors, the gem can be distinguished from synthetic material by optical examination. Such natural samples show sectorial growth zoning, in most cases also associated with polysynthetic twinning and, occasionally, with other forms of growth zoning or twinning (e.g. Dauphiné twinning). These patterns are absent from synthetic amethyst. However, if a Rwandan amethyst has been faceted from only a single untwinned growth sector, the microscopic pattern would show only rhombohedral growth zoning parallel to a single rhombohedral face and no twinning (and thus no Brewster fringes). An analogous pattern is common for synthetic amethyst. Thus, samples with these features would require additional examination, such as by infrared spectroscopy, to identify the nature of the material.

 A future paper by author BW is planned to examine the spectroscopic properties of the Rwandan amethyst. 

This is a summary of an article that originally appeared in The Journal of Gemmology entitled ‘Gem-Quality Amethyst from Rwanda: Optical and Microscopic Properties’ by Karl Schmetzer and Bear Williams 2018/Volume 36/ No. 1 pp. 26-36

Cover Image: The Rwandan amethyst shown here includes a rough piece weighing 101 g, loose faceted gems of 26.62–63.41 ct and an 11.61 ct centre stone that is surrounded by pink sapphires in a 14-carat white gold ring. Courtesy of Steve Moriarty, Moriarty’s Gem Art, Crown Point, Indiana, USA; photo by Jeff Scovil.

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