This OpenSCAD script will generate a Tiffany Etoile style ring with a tension/Bezel/Partial-Bezel set jewel mount for a brilliant (round) cut diamond or similar shiny thing.
Most parameters are user selectable as are the ring profile shapes.

Potentially very useful when combined with the following:
Ring metal cost calculator http://www.thingiverse.com/thing:12589
Diamond size shape weight conversion Spreadsheet http://www.thingiverse.com/thing:12586
Ring Size System Coversion http://www.thingiverse.com/thing:12689
for creating and costing a custom engagement ring or other such tomfoolery.

Todo:
add option to round edges on crown
make gap between 2 sides of crown slightly smaller than required to exert pressure on sides of jewel to hold it in place for proper tension set performance
add complete roman font polygons for engraving
implement hidden compartment

Warning:
Let the right one in

Changes from previous versions:
0.14

• Replaced deprecated import_stl with import
• Added control variable "generate_jewel", if 0 import the jewel from "brilliant1.stl", else try to generate it on-the-fly (this may cause errors as pointed out by mrchriseyes)
  0.13
• Added Simple internal engraving functionality (you will need to create a custom polygonal script though!)
• Finally got the person's ACTUAL ring size!6 6- USA
• Grooves for diamond were too small Need to scale stone by about 1.07 to acount for diameter loss
  0.12
• Added Secondary Crown to make it more similar to Tiffany Etoile Solitaire
• Used Render() and gave option to load the Brilliant cut Diamond shape from external STL file for better previews (the complexity was causing CSG rendering to be disabled)
• Added additional code to ensure the inner surface of the ring is preserved and smooth as crown shapes could poke through, creating sharp edges on inside of ring
• Fixed colours on certain areas
  0.11
• Fixed issue whereby rounded rectangle shape CS appeared slightly translated in Y direction (taken from Ring_band_creation_script_v1.1.scad)
• Added extra cross sections (cs_half_pill, cs_reverse_hemi_ellipse, cs_reverse_rounded_hemi_ellipse taken from Ring_band_creation_script_v1.1.scad)
  0.10
• Round edges on cylinder unched through centre of crwon to back of ring
• Add option to have side supports on crown
  0.9
• Added user variable to control colour and opacity of jewel and metal material
• Can select which type of split you want for the crown (none, rectangular, bowtie, both) by setting with_bowtie_split and with_rectangular_split to 00, 01, 10 or 11 respectively
• Can now choose whether crown is splayed out/tapered or not
• Finally remembered to difference the brilliant cut gem from the ring to make the groove to hold the brilliant cut round jemstone
• Added ability to specify diamond size in Carats to specify girdle diameter, assuming a round cut close to ideal it should be reasonalby accurate (based on data from http://d.neadiamonds.com/diamond-carat). Relationship appears to be close to power or logarithmic curve
• Power Curve size[mm]=6.5065412147(Carat^0.3381325352), confidence value R^2=0.998
• Log Curve size[mm]=1.964367652 Ln(Carat) + 6.6289134546
• Changed most crown variables so that they are influenced by the size of the stone chosen
• De-deprecated some variables
• Added new cross sectional shape cs_rounded_scallopped_rectangle
  0.8
• Added outwards splay/taper to jewel setting based on size of crown face and size of ring
• Identified some redundant variables
• Added additional cross sectional shapes (chamfered rectangle)
• Fixed some CS that resulted in internal diamteres less than expected (cs_rounded_hemi_ellipse, cs_rounded_rectangle, cs_chamfered_rectangle)
  0.7
• Added ring sizes from 4 systems based on linearisation of data found here http://en.wikipedia.org/wiki/Ring_size
• Added acitional cross sectional shapes (rounded rectangle and chamfered rectangle)
  0.6
• Added multiple user selectable cross sections (rect, ellipse, D, chamfered, filleted, scallopped)
  0.5
• Added ability to make a bowtie shaped cutout through the jewel mount and ring, 'cause it looks cool!
• Added ability to color the ring & crown part of the eh... ring!?
  0.4
• Fixed issue with internal diameter of ring not being that specified by user
• Added ability to control width of rectangular split/cutout on jewel mount
• Added ability to control depth that jewel mount overlaps band of ring
• Changed cylindrical cutout in jewel mount to be conical
• Added an actual scalable brilliant cut diamond shape (with ability to give it a "color") within the jewel mount(pretty!)
  0.3
• Changed to pill shaped cross section
• Added rectangular cutout on jewel mount to make split/tension set type fitting, it doesn't line up correctly though :(
• Added commented out/empty calls to functions to enable rest of functionality I desire
  0.2
• I don't think I ever got around to changing this version, please ignore this :p
  0.1
• First attempt
• Makes rectangular cross section ring with hollow cylindrical shaped mount for jewel
• Allows user to define feature size

1. Steal target's rings/get their ring size
2. Acquire jewel carat weight or size (diameter) in mm
3. Decide on cross section type, ring band width, depth and feature size, crown diameter and ring sizing system
4. Modify "user variables" section of script with these values
5. Render your custom ring
   5a. Print in plastic and hope your target cares not for material value
   or
   5b. Print in metal using a service like shapeways and hope your target cares not for hallmarks
   or
   5c. Find reputable jewel merchant/metalsmith/ring maker etc. to bring your item to life professionally so their will be little room for complaints