The Dialogue of Diamonds.
as many foule diamonds as doe contein Samples of every sort of fault & a note of such abatements as an experienced Jeweller would make for every such fault, the same to be expressed in aliquot parts of the whole value, & you must also have a pair of excellent Spectacles for the older sight with a good microscope, & then I conceive you are furnisht with the means of knowing more than most jewelers doe know.
B. I cannot remember all you have said: therfore repeat the same over again in parts, & first concerning the weight.
A. I shal. The general rule concerning weight is this that the price rises in duplicate proportion of the weight, that is to say as the Squares of the weight are one to another or the weight multiplyd by it self. As for Ex.: Suppose a diamond weighing one grain to be worth 20s then a diamond of 2 grains is worth 4l., because the square of two is 4, that is, 2 multiplyd by 2 makes 4; & the diamond of 2 greins is to be paid for as if it weighed 4 & by the same rule a diamond of 3 grains must be reckoned as if [it] weighed 9, because 3 times 3 makes 9, & a diamond of 4 grains is to be reckond as 16, & according to this rule the great Moguls diamond of 1000 grains is reckoned worth a million of pounds Sterling and the Duke of Florences 200000l. Now judge you whether it be safe buying a diamond of 20 grains by the eye without weighing, in which a graine difference in the weight makes about 43l., difference in the price, reckoning the single grain but for 20s.
B. I have one notable & obvious objection against your rule, which is that Lapidarys do use to divide a stone into 2 parts, making according to your rule each half to be but a quarter of the value of the whole & the two halfs after the charge and hazard of dividing to be worth but half what the whole was worth before dividing—answer me that.
A. I doe acknowledge that the rule of weight alone is insufficient, as you have judiciously observed. Wherfore you must come to the next measure which is extent; and extent is chiefly measured by the magnitude of the superficies which the great section of the stone doth make, and by cutting the stone into two parts, if the stone were valued only by the said superficies, the value of the stone cut is doubled, whereas