I attempt to accurately represent and describe cut stones and rough
as to appearance, size, origin and fully disclose any known
treatments in its description. Because you view the images on a
different monitor with different settings, color is particularly fickle
to represent, but I do my best.
TBM
By: Dan Starr
Cutting Instructions
TBM - Designed by Dan Starr © 2007
Angles are for R.I = 1.54
53 Facets  +  16 Girdle Facets  =  69  Total Facets
4-fold mirror-image symmetry
96  Index
L/W = 1.000  T/W = 0.497  U/W = 0.497  P/W = 0.457  C/W = 0.150
Volume/W e3  =  0.213
Cutting Remarks  -  TBM stands for Tunnel Boring Machine. The crown was meant to look like the drill head on one of
those huge machines that drills out tunnels. Take care when roughing in C4: leave enough room for the polishing when meeting
C2.  I roughed in the table prior to polishing C5 and C6 to cut back on the time required to polish them.  If the table doesn't fit
perfectly, then the C5 facets can easily handle the adjustments necessary.
Brightness Table
R.I.
Iso %
Cos %
1.54
89.8
83.5
1.62
92.3
84.3
1.76
88.6
78.5
1.96
86.6
73.7
2.17
86.5
70.4
TBM cut in Blended Bolivian Ametrine
2.95 ct  - 9.7 mm x 6.2 mm
Pavilion
P1
42.40°
96-12-24-36-48-60-72-84                             
Cut to center point.                                                                                            
P2
41.60°
06-18-30-42-54-66-78-90                             
Cut to the same center point as P1.                                                                   
P3(G)
90.00°
96-12-24-36-48-60-72-84                              
Cut to meet P1 and P2 to form girdle.                                                                
P4(G)
90.00°
06-18-30-42-54-66-78-90                             
Cut to meet P1.                                                                                                  
Crown
C1
31.71°
96-24-48-72                                                   
Cut to form girdle.                                                                                              
C2
35.90°
06-18-30-42-54-66-78-90                              
Cut to meet C1 and form girdle.                                                                         
C3
33.75°
12-36-60-84                                                   
Cut to meet C2 and form girdle.                                                                         
C4
27.47°
01-23-25-47-49-71-73-95                              
Cut to meet C2.                                                                                                  
C5
24.25°
02-22-26-46-50-70-74-94                              
Cut a step.                                                                                                          
C6
30.66°
12-36-60-84                                                   
Cut to meet C4.                                                                                                  
T

Table                                                                   
Cut to meet C4.                                                                                                  
Cutting Instructions
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