in the value of the base. A base of $3 would give a value of $75 for a
five-grain pearl, or $15 per grain, while a $10 base would make the
value $50 per grain, or $250.
method of estimating pearls by squaring their weights has been credited
by many authors to David Jeffries, who published an interesting
treatise on diamonds and pearls in 1750-1753. It has also been credited
to Tavernier, the oriental traveler of the middle of the seventeenth
century. We have, however, traced this method back to Anselmus de Boot,
in his treatise on precious stones, dated 1609. Before this date we
have not been able to find any mention of the computation of the value
of diamonds and pearls by squaring their weight and multiplying the
product by a base of a franc, guilder, crown, dollar, or of many
dollars, as would be necessary at present. It is probable, however,
that this system is of oriental origin and it may have come to Europe
through some of the oriental traders, with the precious stones, as did
the use of the carat.
Boot makes the carat (four grains) his unit of comparison, increasing
his base value by one third for pearls weighing eleven carats
(forty-four grains) or over. In Pio Naldi's treatise, published in
Bologna in 1791, the unit is the grain, the base being the fourth part
of the value of four pearls weighing together one carat. Naldi, also,
increases his base value making it 1-1/2 lire ($.30) for pearls weighing less than ten grains, and 2-1/2 lire ($.50) for those weighing twenty grains and upward.
A curious method of valuing pearls by their weight is shown in a treatise by Buteo, published in 1554.1
The writer states that a pearl weighing two carats was valued at 5 gold
crowns ; one of four carats at 25 crowns ; and so on, the price
increasing fivefold when the weight was doubled. The intermediate
figures were obtained by computing the proportional mean of any two
known weights and values. For example : 8 x 4 = 32, the square root of
which is 5.656. Now, the value of a four-carat pearl is 25 and that of
an eight-carat pearl 125 crowns, and 125 x 25 = 3125, the square root
being 55.9; hence a pearl weighing 5.656 carats was worth 55.9 crowns.
base value of a necklace can be determined in the following way. Should
the center pearl weigh 25 grains, multiply 25 by 25 ; the result is 625
; then, take the next two, three, or four pearls, as many as are of
approximately the same weight, add their weights together, multiply the
resulting figure by itself and divide the product by the number of
pearls in the group. Proceed in exactly the same way with the remainder
of the necklace, always grouping the pearls so that there shall not be
a considerable difference in weight between the smallest and the
largest pearl, and then add together the figures obtained for
1Buteonis, "Opera Geometries," Lugduni, 1554, pp. 88-96.