Retail jewellers rarely deal in loose stones so it is important for people who work in retail (as well as informed customers) to be able to estimate the proportions of diamonds that are mounted in jewellery. This post reviews some of the key formulas professionals use to assess the proportions of mounted diamonds.
This first thing to understand about mounted diamonds is that the mounting can hide inclusions. Mountings such as prongs can also interfere with lighting and cause reflections that can be misleading. To see what’s under a prong, find the prong’s reflection on the opposite side and rock the diamond back and forth.
A mounting can inhibit the color of a stone by blocking light to the pavilion, or enhance it by reflecting color off the metal. Yellow diamonds are commonly mounted in yellow gold to enhance their color. To assess a mounted stone’s color, hold the stone face-up under 10X magnification and compare it to master stones in the same color range. Hold the two stones close together and try not to let them touch. Repeat the process until a match is confirmed.
Evaluating measurements and cut
Mounted Diamonds can be difficult to measure. This is especially true if the stone is bezel sett, leaving the girdle inaccessible. If the girdle can’t be reached, the diameter can be estimated by following the bezel facets and guessing where they come to a point. If the Diamond is round, take 4 measurements and average to get the diameter.
Of all the variations a diamond can have, girdle thickness has the biggest effect on weight. To account for this, weight correction must be factored into the estimate. A weight correction of 1-10% can be selected based on Girdle thickness. Below are the formulas for estimating the weight of popular diamond cuts.
- Round Brilliant - Estimated weight = average girdle diameter2* × depth × 0.0061 × weight correction
- Oval Brilliant - Estimated weight = average girdle diameter2* × depth × 0.0062 × weight correction
- Heart - Estimated weight = length × width × depth × 0.0059 × weight correction
- Princess Cut - Estimated weight = length × width × depth × 0.0083 × weight correction
- Baguette - Estimated weight = length × width × depth × 0.00915
- Emerald Cut - Estimated weight = length × width × depth × adjustment factor × weight correction
- Marquise - Estimated weight = length × width × depth × adjustment factor × weight correction
- Pear - Estimated weight = length × width × depth × adjustment factor × weight correction