Gemstone Grading Cut & Proportion

A clean gem that is then cut to good proportions will return a large percentage of light to the eye. A poorly cut gem will normally have:

  • Large areas of windowing
  • Large areas of extinction
  • An imbalance of brilliance
  • No brilliance at all
  • All of the above

Good gem cutters cut to optimize a gem’s beauty and its carat weight. And sometimes that is a tough balancing act.

Gemstones usually sell by the carat. Yet a more beautifully faceted and polished gemstone will fetch more per carat.

One problem is that so much ruby, sapphire and spinel is native or jungle cut where carat weight is the sole compelling factor.

Factors Considered in Cutting Gemstones

  • Carat weight – without sacrificing the gem’s appeal
  • Color – by properly orienting the crystal while considering where the gem’s color center is located
  • Brilliance – without sacrificing weight
  • Clarity – by hiding inclusions under crown facets, etc
  • Spread – which is the gem’s perceived size, without windowing the gem Symmetry – while preserving carat weight

The importance of a well proportioned Cut is greater than many grading systems assign to it and should be more strongly considered in valuing colored gems.

Gemstone Cut Evaluation

  • Depth : Width Ratio – ratio of depth to width / diameter, expressed as a [%] percentage
  • The girdle of a gem, if excessively thin is easier to damage. If excessively thick, will add weight that does not contribute to a gem’s appeal, plus a thick girdle makes setting it difficult
  • Girdle Appearance – a wavy girdle detracts from a gem’s appeal, and will mean that crown facet alignment is not correct
  • Length : Width Ratio – acceptable ratios vary depending on the cut, but a ratio greater than 1.5 should be considered excessive. An exception could be for marquise cuts which might approach 2
  • Crown : pavilion Ratio – viewed in a profile, should fall between 1:3 and 1:5
  • Symmetry – the balance of a gem. If a gem is cut in half, each half should be a mirror image of the other
  • Finish of a gemstone
    • Poor finishing leaving nicks or pits in a gem’s surface
    • Facets should have no visible polishing marks
    • Facets should show high surface luster
    • Facet junctions should be crisp, with no rounding
    • Facets should align perfectly when viewed without magnification
    • Girdles should be polished

Establishing a Basic Cut Grade for Gems

Start with 10 Points, then subtract 1/2 point for each of the following undesirable characteristics observed in the gem.

  • Poor length to width ratio
  • Poor crown to pavilion ratio
  • Poor table to crown ratio
  • Excessively thick or thin girdle
  • Obvious variation in girdle thickness
  • Wavy Girdle Plane
  • Unsymmetrical profile outline
  • Unsymmetrical face up outline
  • Polishing lines
  • Facet misalignment
  • Large culet/keel (looks like a facet)
  • Excessive bulge
  • Excessively long keel Line
  • Off center culet or keel line
  • Overall poor surface luster
  • Tilted table

Yes, you can end up with a negative result, but when approaching a grade of “1”, the gem probably shouldn’t even be graded.

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