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Ch. 10: Gem Cutting Engraving

Ch. 10: Gem Cutting Engraving Page of 296 Ch. 10: Gem Cutting Engraving Text size:minus plus Restore normal size   Mail page  Print this page
260
PRECIOUS STONES.
In Fig. 103 let the line AB which joins the two opposite summits, be divided into six equal parts, and let a plane perpendicular to the line AB pass through the second division from the upper point, and another plane also at right angles to AB pass through the first division from the lower point, then a small pyramid will be detached from each extremity, and there will remain the solid repre­sented by Fig. 104.
These are the proportions in which the axis is cut by the French lapidaries, but English lapidaries usually cut five-eighteenths from the upper pyra­mid, and one-eighteenth from the lower. The upper and larger plane surface is called the table; the lower is named the collet (French, culasse).
The four superior edges or ribs, and the four in­ferior edges, are then removed in such a way that the table and the collet are circumscribed by regular octagons, as represented by Fig. 105.
Ch. 10: Gem Cutting Engraving Page of 296 Ch. 10: Gem Cutting Engraving
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