Weighty Considerations 
by Richard Cartier FGA

The weights of individual gemstones, or the total weight of a group of gemstones, should be expressed in carats (symbol ct) to two and only two decimal places.  It is appropriate to quote carat weight to three decimal places as additional information or clarification appended to the official two decimal place carat weight.  For example “weighing by scale 1.00^(0.999) carat” or “weighing by scale 1.00^(1.004) carat” is acceptable, but the official carat weight should be to two and only two decimal places.

Requirements for rounding of the carat weight vary according to jurisdiction, and it is not always to the nearest 0.01 ct.  The International Confederation of Jewellery, Silverware, Diamonds, Pearls and Stones (CIBJO) and most diamond clubs require that the third decimal place be ignored except when it is a nine, in which case the second decimal place numeral may be rounded up.  In discussing this issue of rounding with dealers in fine coloured gemstones in Toronto, New York, and London the consensus seemed to be that ignoring the third decimal place unless it is a nine would be the preferred rounding convention.  This approach is recommended because it guarantees that the client will receive full measure.

The great advantage of the two-decimal-place carat convention is that it allows for very little misunderstanding of specified carat weights.  By the above convention and rounding standards the stone that shows 0.025 on the weigh scale must be called 0.02 ct, which could never be mistaken for a quarter of a carat instead of a fortieth.  The stone that shows 0.125 on the scale must be called 0.12 ct and so could never be mistaken for over one carat.

Specify “in total” when quoting the weight of more than one stone.  Also indicate “by scale” if the stones were actually weighed, or “approximately” if estimated or if reporting a purported actual weight that was confirmed from the stone's dimensions is likely to be accurate.

This convention of reporting two decimal places for carat weight should be used without regard to precision of weight estimation.  If using standard formulae to calculate the approximate weight of a mounted stone from its dimensions and assumed specific gravity and, after adjustments for girdle thickness, bulge factor, and crown height, the number on the calculator is 5.0429825 report “weighing approximately 5.04 carats” because this is a best effort at estimating the official two decimal place carat weight, and has been declared as an estimation.

In circumstances when it is not possible to accurately measure or make a reasonable estimation of the dimensions of a stone “weighing approximately five carats,” indicates a general approximation but if reporting in numerals specify two decimal places according to the convention.

When the weight of a stone is on the cusp between two price groups it must be established which price group is being worked with to set the value.  Undoubtedly the stone “weighing approximately 5.04 carats,” may just as easily have an actual weight of 4.96 ct or 5.12 ct, and the 4.96 ct stone would have a lower per carat value.  The job of appraiser is to formulate an opinion.  A report that says “weighing approximately 5.04 ct,” clearly indicates which price group was assumed.  A report saying “approximately five carats,” and offering a single value may leave doubt about which price group was used and the opinion is less rigorous.  It would be better to add “the valuation assumes a full five carats” than to leave it unclear.  Do not ever use an average between price groups for weight ranges, a stone is either in one weight range or the next, not half way between the two.  The alternative to choosing one or the other is to report “approximately five carats” and offer two values, one for short of five carats, the other for a full five-carat stone.

Do not put a period after ct or pluralize it.  Consider ct to be a symbol.  As a symbol it requires neither pluralization nor special punctuation.

Furthermore, do not pluralize the word carat unless the weight quoted is equal to or greater than 1.01 ct even if the third decimal place would show a weight greater than unity.  Acceptable examples would be “2.18 carats”, “2.18 ct”, “1.01 ct”, “1.01 carats” or “1.00^(1.008) carat”.  Note that “0.13 carats” is quite unacceptable, as is “1.00 carats”; while “2.18 cts” shows a mild lack of understanding about use of symbols with the redundant s.

Always place a zero before the decimal point of a carat weight when the weight is less than 1.00 carat (e.g. not “.27 ct” but rather “0.27 ct”).

It has become conventional throughout the jewellery industry that carat weights of less than one carat may be spoken of as a one or two digit numeral expressed in words and followed by the word point(s) to indicate that these numerals in the carat weight are to the right of the decimal point.  For example a stone weighing 0.21 ct may be said to be a twenty-one point stone or 0.07 ct may be referred to as seven points.  This results from the curious difference between written and spoken language when discussing numbers.  “A twenty-one point stone” is quicker and easier to say than “a stone weighing zero point two one carat”, while “0.21 ct” is quicker and easier to write.   Similarly “seven points” is much quicker and easier to say than “zero point zero seven carat” while “0.07 ct” is quicker and easier to write.  In each of these cases the information conveyed is a clear unambiguous representation of the carat weight of each stone.  The word point in each case refers to the decimal point in the carat weight and not to another unit of weight.  We are always talking about carat weight, not “point weight”.  This is quite clear if someone says “The carat weight is twelve points.”

In discussing carat weight the word point should not be abbreviated in text and should be in combination with the number section of the carat weight in word format only, not numerals (e.g.:  write  “0.12 ct” or “twelve points” but not “12 points”, nor “0.12 points”, nor “0.12 pt.”, nor “twelve pt.”).

The reason this use of the word point works so well and is not misunderstood is because of the two and only two decimal place convention for carat weight.  To the right of the decimal point there are always two digits.  Misunderstanding develops only when "point" is considered a unit of weight rather than a way of expressing the carat weight verbally.

Years ago there was a national Canadian newspaper advertisement that had jewellers telephoning one another to ask if they had seen “that crazy ad for half carat diamond ear studs at a ridiculous price”.  What the advertising offered was a pair of “0.50 pt total weight” diamond ear studs.  The vendor claimed that there was nothing misleading about his special sale because all the items he was selling contained well in excess of the advertised total weight of half a point.  One pair of those “earrings” seen by the author contained two “promotional” quality (i.e. rejection quality that would take a lot of promoting to sell to anyone) single cut diamonds weighing in total approximately three points set in gold plated base metal.  Remember this sheer nonsense of “fifty points of a point” as a reminder that point is not a unit of weight, it is a way of expressing carat weight in words.  Get the point!

Extremely small diamonds are a nomenclature nuisance and are rarely encountered singly in articles of jewellery worthy of being appraised.  If a diamond weighing under 0.01 ct must be described specify that the weight is “less than 0.01 carat”, additional details of weight being neither required nor prohibited, but better left unspecified.

For information and calculation purposes small diamond melee information may be recorded on the work sheet in a fraction format with the numerator being the number of stones and the denominator how many of them it would take to make up a full carat of weight.  Any written report, however, should specify the total weight of a given number of like stones in standard two decimal place format.  As an example, given six diamonds, forty of which would weigh one carat, record on the work sheet 6/40 ct and report six diamonds weighing in total 0.15 carat.  Twelve mounted single cut diamonds each being 0.9 mm in diameter could be recorded as “12/240 ct 8/8” and reported as twelve single cut diamonds weighing in total approximately 0.05 carat.  In the latter case “8/8” is a short hand way of writing single cut^[1], the style of cut having eight main facets above and eight main facets below the girdle.

When reporting on similar stones in a group, don’t give the count and the weight of one of the stones.  Give the count and the total weight of all the stones together.

The total-weight approach means that one diamond weighing 0.20^(0.206) ct together with a matching diamond weighing 0.20^(0.208) ct is described as two diamonds having a total weight of 0.41 ct, which is more accurate than 0.20 ct (according to the rounding rule) each.  This eliminates temptation to specify carat weight of small melee in (foolish and) undesirable “fractions-of-a-point”!

Stones that are considered together as a group in arriving at a perceived cost should be described together as a group giving the count and total weight.  For example, say “a pair of 14 Karat white gold six-prong buttercup style ear studs containing two brilliant cut diamonds measuring … weighing in total approximately 0.41 ct, of G-H colour, SI clarity, and good make.”

Stones that are considered separately to arrive at a perceived cost should be individually described with the weight of each individual stone reported.  For example, say “a pair … ear studs … containing one brilliant cut diamond … weighing approximately 0.20 ct, of E-F colour, VS clarity, and good make; and one brilliant cut diamond … weighing approximately 0.20 ct, of H-I colour, SI clarity, and good make.”

It is better to avoid using fraction format (such as “half carat”) to describe carat weight.  If there is some reason to prefer using such a format to describe the weight of gemstones be sure that the weight of the reported stone(s) will meet or exceed the equivalent decimal carat weight notation.

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