GemCAD: Computer-Aided Design Software for Faceting
GemCAD: Computer-Aided Design Software for Faceting
The industry-standard tool for modelling, verifying, and publishing faceting diagrams
GemCAD is a computer-aided design programme purpose-built for the creation and verification of faceting diagrams. Developed by Robert Strickland and first released in the early 1990s, it has become the de facto standard among amateur and professional faceters in the English-speaking world, particularly within the United States Faceters Guild (USFG) community. The software allows a cutter to model a proposed design in three dimensions before a single facet is ground, calculating meet-point geometry, crown and pavilion angles, and overall light performance with a precision that hand-drafted diagrams cannot match.
What GemCAD Does
At its core, GemCAD operates on the same geometric principles that underpin any faceted stone: a series of flat polished faces, each defined by its angle relative to the girdle plane and its rotational position on an index wheel. The user inputs these parameters — typically expressed as an angle in degrees and an index number — and the software constructs a wire-frame three-dimensional model of the resulting stone. From this model it derives several critical outputs:
- Meet-point verification: GemCAD checks whether facet junctions close precisely at a single point, flagging any geometric inconsistencies that would produce unsightly ridges or open junctions on the finished stone.
- Printable diagrams: The programme generates standardised diagrams showing each facet tier in sequence, with angle, index, and cutting order clearly annotated. These diagrams follow conventions familiar to any USFG-trained cutter and can be reproduced on different machines with different index gears.
- Reflectance modelling: A ray-tracing display allows the designer to visualise how light will behave within the stone — identifying potential windowing (where the eye looks straight through the pavilion) or extinction (dark zones caused by steep angles) before cutting begins.
- Refractive index input: Because optimal angles differ between species, GemCAD accepts a refractive index value for the material being modelled. A design optimised for quartz (RI approximately 1.55) will differ meaningfully from one intended for corundum (RI approximately 1.76), and the software accommodates this throughout its calculations.
File Format and Design Libraries
GemCAD uses a plain-text file format with the extension .gem, which has proved durable and widely shareable. Thousands of published designs in this format are freely available through the USFG, the Gemological Institute of America's faceting resources, and independent cutter communities online. This open ecosystem means that a cutter can download a design, adjust the refractive index for their specific rough material, and verify meet-point accuracy before committing to the wheel — a workflow that has substantially lowered the barrier to cutting complex original designs.
The programme also supports the GemRay companion utility, which extends the optical modelling capabilities, and designs can be exported for use with certain computer-numerically-controlled (CNC) faceting machines, bridging hand-cutting tradition and automated precision manufacturing.
Place in the Faceting Community
GemCAD occupies an unusual position in the lapidary world: it is simultaneously a professional design tool and an accessible educational resource. Competition faceters use it to develop and refine original designs for USFG and AFMS (American Federation of Mineralogical Societies) judged events, where meet-point precision is a scored criterion. Teachers use it to explain the geometry of standard cuts — the round brilliant, the emerald cut, the Portuguese — in a visual, interactive way that static diagrams cannot replicate. And collectors of faceting diagrams treat the accumulated .gem file libraries as a kind of open-source pattern archive, analogous to the published diagram books of the pre-digital era.
While newer browser-based and parametric design tools have emerged in recent years, GemCAD's longevity reflects both the stability of its file format and the depth of the design library built around it. For any serious student of faceting geometry, familiarity with GemCAD remains a practical necessity.