Gemstone Windows and Dark Patches: What to Look for in a Cut Gem

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AI-generated educational illustration. Conceptual only; not a photograph of an actual specimen or location. A coloured gem may have a pale, see-through centre or areas that look almost black. Both affect its appearance, but they do not have the same cause. What is a window? A window is an area where light passes through the gem instead of being returned towards the viewer. You may see the background through it, especially near the centre. Cutting angles and the material's refractive index—how it bends light—help determine whether this happens. There is no single ideal depth for every gemstone. Why dark patches need a closer look Darkness can have several causes. The material may absorb a lot of light. Some facets may reflect dark surroundings or the observer. Extinction is another cause, involving light that is not returned to the viewer after internal reflection. For that reason, calling every black patch “extinction” can be misleading. A simple viewing routine Look at the gem fac...
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Quartz

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Quartz is the second-most abundant mineral in the Earth’s continental crust, after feldspar. It is made up of a continuous framework of SiO4 silicon-oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall formula SiO2. There are many different varieties of quartz, several of which are semi-precious gemstones. Throughout the world, varieties of quartz have been since antiquity the most commonly used minerals in the making of jewellery and hardstone carvings.

The quartz group includes a number of different species/varieties. There are two categories crystalline quartz and cryptocrystalline quartz (chalcedony).

Source: en.wikipedia.org/wiki/Quartz

Cause of Color: Rose Quartz: Charge transfer reaction (Ti4+ and Fe2+) as a result of irradiation.
Chemical Composition : 

Silica (Silicon Dioxide) SiO2

Crystal System / Forms : 

Trigonal System

  • Prismatic with pyramidal terminations (rhombohedron, trapezohedron, trigonal, pyramid)
  • Horizontal striations on the prism face, rhomb shaped on the rhombohedral face, ‘V’ shaped markings in twinned crystals.
  • Most quartz is commonly twinned.
  • Types of twinning include Dauphine, Brazil and Japan law twinning.
  • Exhibits both contact and interpenetrant twinning.
Cuts & Uses : 

Facetted cuts, cabochon, carvings, beads, etc.

Dispersion : 0.013
Hardness : 7
Lustre : Vitreous.
Magnification : 

Liquid and two-phase, colour zoning, negative crystals, zebra-stripe fingerprints (structural), crystal inclusions, green fuchsite mica flakes/platelets in aventurine quartz, rutile/tourmaline needles in sagenitic quartz. Brazil law twinning is seen in natural quartz.

Optic Character : 

Anisotropic, D.R.; Uniaxial positive. May exhibit a bull’s eye optic figure (quartz rotates the plane of polarisation parallel to the c-axis).

Pleochroism: Dichroism - weak to moderate.
Refractive Index / Birefringence : 

1.544 – 1.553 / 0.009. Range: 1.535 – 1.560

Sources : 

Brazil, India, South Africa, Namibia, Madagascar, Zambia, Sri Lanka.

Specific Tests :
  • Piezoelectric: develops an electric charge when pressure is applied.
  • Pyroelectricity: develops an electrical charge when heated.
  • Diasterism in star variety (star visible in reflected and transmitted light).
Spectrum : 

Green aventurine: bands at 682nm, 649nm

Synthesis : 

Hydrothermal process:

  • Colours – colourless, pink, blue, green, etc.
  • Identification: type of twinning, seed plate, breadcrumb inclusions, Raman / infra-red spectroscopy.

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