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Gemstones, Minerals & Fossils
Lapidary
Will changing angles in designs fractionally really matter?
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<blockquote data-quote="Lefty" data-source="post: 286055" data-attributes="member: 2976"><p>I have rounded fractions that were close to whole numbers with no detriment whatsoever to the stone. </p><p></p><p>I have altered angles by a couple of degrees with no noticeable impact - though I would be careful lowering pavilion angles in case you fall foul of the critical angle - the angle at which light entering the stone through the top will not be reflected back as brilliance but will simply pass out through the bottom. Critical angles vary with different materials - quartz is about 40.5 degrees, feldspar (labradorite) about 41.5 (I think). Sapphire is much lower at about 34.5 (again, if memory serves correct) - the lower the materials critical angle, the more leeway you have to shallow the stone....to a point. Get too close and you'll get "tilt windowing" - I once cut a piece of smoky quartz in a design with angles meant for materials of a higher refractive index, the result was a flash of brilliance in one half of the stone and in the other half I could look right through it and see my fingers and the gem grabbers underneath.</p><p></p><p>Even with sapphire some say never take the culet angle below 39 degrees - I've never gotten quite that close but 40 degrees still produced a nice, brilliant stone.</p><p></p><p>The crown angles are somewhat more forgiving than the pavilion angles since it is the pavilion that is the primary reflector.</p><p></p><p>As long as you remember what you rounded them to - write them down - and steer clear of the critical angle when cutting the pavilion, small rounding changes to angles should make little or no difference.</p></blockquote><p></p>
[QUOTE="Lefty, post: 286055, member: 2976"] I have rounded fractions that were close to whole numbers with no detriment whatsoever to the stone. I have altered angles by a couple of degrees with no noticeable impact - though I would be careful lowering pavilion angles in case you fall foul of the critical angle - the angle at which light entering the stone through the top will not be reflected back as brilliance but will simply pass out through the bottom. Critical angles vary with different materials - quartz is about 40.5 degrees, feldspar (labradorite) about 41.5 (I think). Sapphire is much lower at about 34.5 (again, if memory serves correct) - the lower the materials critical angle, the more leeway you have to shallow the stone....to a point. Get too close and you'll get "tilt windowing" - I once cut a piece of smoky quartz in a design with angles meant for materials of a higher refractive index, the result was a flash of brilliance in one half of the stone and in the other half I could look right through it and see my fingers and the gem grabbers underneath. Even with sapphire some say never take the culet angle below 39 degrees - I've never gotten quite that close but 40 degrees still produced a nice, brilliant stone. The crown angles are somewhat more forgiving than the pavilion angles since it is the pavilion that is the primary reflector. As long as you remember what you rounded them to - write them down - and steer clear of the critical angle when cutting the pavilion, small rounding changes to angles should make little or no difference. [/QUOTE]
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Gemstones, Minerals & Fossils
Lapidary
Will changing angles in designs fractionally really matter?
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