Blue Light Special

Blue Light Special
Designed by Bob Keller 07-00 Bob Keller
Angles for R.I. = 1.54 111 facets + 20 facets on girdle = 131
20-fold symmetry 80 index
L/W = 1.000 T/W = 0.441 T/L = 0.441 P/W = 0.435 C/W = 0.128 H/W = (P+C)/W+0.02 = 0.583 P/H = 0.746 C/H = 0.220
Vol./W^3 = 0.179 Brightness at 0° tilt COS = 91.2 ISO = 94.0
Brightness at 5° tilt COS = 82.9 ISO = 87.3
Pavilion 80 Index
1 41.70 02-06-10-14-18-22-26-30-34-38-42-46-50-54-58-62-66-70-74-78 Cut to center point
G2 90.00 02-06-10-14-18-22-26-30-34-38-42-46-50-54-58-62-66-70-74-78 Match 1, set size, form level lower girdle line
3 41.00 04-12-20-28-36-44-52-60-68-76 Meet 1,G2
Crown 80 Index
A 27.50 02-06-10-14-18-22-26-30-34-38-42-46-50-54-58-62-66-70-74-78 Match G2, form level upper girdle line
B 25.50 80-04-08-12-16-20-24-28-32-36-40-44-48-52-56-60-64-68-72-76 Meet A,G2
C 23.50 02-06-10-14-18-22-26-30-34-38-42-46-50-54-58-62-66-70-74-78 Meet A,B
D 21.50 80-04-08-12-16-20-24-28-32-36-40-44-48-52-56-60-64-68-72-76 Meet B,C
T 0.00 Table Meet C,D

GemCad Ray Traces for RI = 1.54
Random Model Cosine Model ISO Model
RI = 1.54 with 5° X-Y Tilt
Random Model Cosine Model ISO Model

GemCad users can download a .gem design file for Blue Light Special: bluelit2.gem.

I designed the Blue Light Special for a larger stone cut from blue synthetic quartz of medium color saturation. It employs vigesimal (20-sided) symmetry to scale up the facet count of the basic brilliant pavilion. The increased symmetry and spiked mains work to drive the break angles towards that of the pavilion mains, which has a brightness increasing effect. The crown pattern follows that of the Portugese cut, but with a reduced number of courses, for a total facet count of 131. You can cut this design using a 40, 80 or 120 index. A cutting chart for a 120 index is provided below.

Larger stones cut from more than lightly saturated material can lose a good deal of light due to absorption. This design couples a shallow pavilion (41° mains and 41.7° breaks) and shallow crown (C/H = 22%) to keep the ray paths relatively short and achieve high brightness to help compensate for the light lost to absorption.

If you're comfortable with an SRB you shouldn't have too much trouble with this design from a cutting point of view, although you may need to do a little post-table touching up on the crown C and D facets, as there are 20 of them meeting the table. Due to machine eccentricities and cumulative errors in your cutting, you are unlikely to get a perfect intersection with all 20 of those meets when you cut in the table.

I've come to appreciate that brighter is not always better and I wouldn't use this design on clear quartz or lightly saturated material. Pale and nearly transparent materials don't suffer much from absorption and if you cut them too bright it washes out rather than enhances their color. To adapt these planviews to lightly saturated material I would probably use 42° mains on the pavilion for quartz, increase the crown angles, and perhaps add another course of crown facets if the stone is large enough to carry them to provide a more arched crown contour and deepen it. A stone large enough to wear this design is going to have relatively good dispersion over it's smaller kin anyway, but higher angles and a deeper crown will deepen and enhance the color of lightly saturated materials, produce a better play of reflections, and better dispersion yet.

If you cut a Blue Light Special please be sure and let me know what you've cut and how you made out with it.

Pavilion 120 Index
1 41.70 003-009-015-021-027-033-039-045-051-057-063-069-075-081-087-093-099-105-111-117 Cut to center point
G2 90.00 003-009-015-021-027-033-039-045-051-057-063-069-075-081-087-093-099-105-111-117 Match 1, set size, form level lower girdle line
3 41.00 006-018-030-042-054-066-078-090-102-114 Meet 1,G2
Crown 120 Index
A 27.50 003-009-015-021-027-033-039-045-051-057-063-069-075-081-087-093-099-105-111-117 Match G2, form level upper girdle line
B 25.50 120-006-012-018-024-030-036-042-048-054-060-066-072-078-084-090-096-102-108-114 Meet A,G2
C 23.50 003-009-015-021-027-033-039-045-051-057-063-069-075-081-087-093-099-105-111-117 Meet A,B
D 21.50 120-006-012-018-024-030-036-042-048-054-060-066-072-078-084-090-096-102-108-114 Meet B,C
T 0.00 Table Meet C,D

Index of Gem Designs

Perfect Transfer Index

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© Bob's Rock Shop Bob Keller