Hi Albert,

The most relevant relationship for your Portuguese is Height/Width, which is the ratio H/W. That ratio is given in your diagram to be:

H/W = .827

CZ is isometric, so you don't need to be concerned with orientation of the optical axis. Assuming your CZ is block shaped, you will get the best yield by turning the block so that it is 13.5mm wide, 14.4mm long and 12.3mm high (thick).

The Portuguese is a round cut, so orienting the rough that way, the maximum possible width (diameter) of the stone will be 13.5mm. Will 12.3mm be high enough?

A little basic algebra is used to solve for the height of the finished stone given the width.

H/W = .827
H = .827 * W
H = .827 * 13.5mm
H = 11.1645

Since your rough is 12.3mm high and a 13.5mm wide stone will be 11.1645 high, your rough is:

12.3 - 11.1645 = 1.1355 mm higher (thicker) than the minimum required to cut the stone to a 13.5mm width.

Your rough may be irregularly shaped or have flaws that you need to cut out or around, your centering of the rough on the dop may not be perfect, you will lose some material during the cutting and polishing process etc., so you will probably not be able to actually get a 13.5mm wide stone out of it.

But the relationship H/W = .827 still applies and can be used to find the minimum height (thickness) required to cut that particular design at any given width.

If the documentation of the design you are cutting provides the Vol./W^3 parameter, you can use my online gem weight calculator to see what it will weigh at any given width:
http://www.rockhounds.com/rockshop/gem_designs/gem_weight_form.shtml

You should consider joining us on the United States Faceters Guild discussion list:
http://www.usfacetersguild.org/faceters_list.shtml

From Bob Keller - December 26, 2009 at 01:31:56

Message: 68646