Look at a faceted gem design without opening the program that cut it.
⬇ Download GCSViewer.exe —
Windows, nothing to install. Double-click a .gcs or .gem file and the
stone is on screen: three shaded views and the cutting instructions.
If you open designs all day, take the folder build from the
releases page instead —
it starts in a third of the time (427 ms against 1,534 ms), because the single
file unpacks itself into %TEMP% on every launch.
docs/demo.gcs, opened in the viewer. The third panel is live: drag to turn
the stone, arrow keys to tip it.
Faceting design files are geometry, not pictures, so a folder of them is a folder you cannot see — and finding one stone means opening candidates in a design program one at a time. This makes them behave like photos: double-click one, arrow through the folder, close it.
Gem Cut Studio, by Rej Poirier, is design
software for faceted gemstones — "gem design, in real-time", in its own words.
You work in the terms a faceter actually cuts to — tiers, angles, index gear —
and see the finished stone rendered as you go, so a design can be judged on
screen before anything is ground against a lap. Its designs are the .gcs
files this viewer opens, and the tier / angle / index table under the renders
is the same vocabulary as its cutting-sequence panel.
GCS Viewer is an independent tool. It is not affiliated with Gem Cut Studio or GemCad, it only reads their files, and it never modifies what it opens.
Three orthographic, flat-shaded panels, tinted with the material colour stored in the file:
| Panel | View |
|---|---|
| Table (top) | straight down the optic axis |
| Side | the girdle profile |
| 3/4 view | angled — drag to turn it, arrow keys to tip it |
Below them the cutting sequence — tier, angle, index list and the instruction the file carries — grouped into Pavilion and Crown.
Angle, section and index list are derived from the geometry rather than read from the file, so they appear even for designs that store no instructions. Index positions come from each facet normal against the gear in the file (96 by default): a normal at 22.5° on a 96 gear is index 06.
A panel with nothing facing it says so, which is not always a fault — a preform has no crown yet, so there is genuinely nothing to draw from above.
Rendering is deliberately flat and matte: a painter's algorithm with back-face culling, one fixed light and a specular highlight. No refraction, no dispersion. Flat shading shows the facets, and a meet that does not meet stays visible instead of disappearing into a sparkle.
.gcs |
Gemcut Studio XML. Facets, tiers, instructions, material colour. |
.gem |
GemCad binary — reverse-engineered, no published spec. |
The .gem reader recovers the title and the designer's notes from a trailing
block of length-prefixed strings, and prints them along the foot of the sheet,
since that is often all a .gem says about itself. Face normals are recomputed
with Newell's method, because the stored ones cannot be relied on.
There is no .asc support, by design.
| Key | Does |
|---|---|
| ← → | previous / next design in the folder, wrapping, in Explorer's sort order |
| drag | turn the 3/4 view left and right |
| ↑ ↓ | tip the 3/4 view up and down |
| I | show / hide the instructions table |
| G | grayscale |
| L | tier labels |
| S | save the sheet as a PNG next to the design |
| R | reset the 3/4 angle |
| Esc / Q | close |
Files it cannot read are skipped while arrowing, rather than stopping the walk.
GCSViewer.exe needs nothing installed — no Python, no numpy, no Pillow.
-
Put it somewhere permanent, e.g.
C:\Tools\GCSViewer\. -
Register it, from that folder:
powershell -ExecutionPolicy Bypass -File .\Install-GcsViewer.ps1
That also gives .gcs and .gem files a gem icon instead of Explorer's blank
page, and adds a Start Menu entry which asks for a design when launched without
one.
Windows lets only you set a default handler, not the program itself — the setting is hash-protected so that applications cannot claim file types behind your back.
Right-click a .gcs file → Open with → Choose another app → GCS
Viewer → Always, and repeat once on a .gem file. If Always is
greyed out, select the app first; if GCS Viewer is not listed, scroll to the
bottom for Choose an app on your PC and browse to GCSViewer.exe.
Gem Cut Studio stays available on both types through Open with.
If double-click shows the picker and choosing the viewer does nothing, re-run
Install-GcsViewer.ps1 — versions 1.0.19 to 1.0.25 registered the file type
without a default action, and re-running repairs it.
To check the registration without opening dialogs:
powershell -ExecutionPolicy Bypass -File .\scripts\Show-OpenWithList.ps1It prints what Windows would offer for both extensions, whether a double-click resolves to a program that exists, and any stale entries left behind by apps that were moved or uninstalled.
If you move the install, re-run the installer from the new location. Any default you explicitly set has to be picked again, because Windows ties a saved choice to the app as it was registered when you chose it.
INSTALL.md is the version to hand to somebody else.
This is how to use it on Mac or Linux. The download is a Windows executable, but the program is plain Python and runs anywhere numpy, Pillow and tkinter do. There is no double-click integration off Windows, so you pass the design on the command line; everything else — the three views, the cutting table, the keys — is the same.
pip install -r requirements.txtpython gcs_viewer.py "stone.gcs" # the window
python gcs_viewer.py "stone.gcs" --save # write stone_views.png beside it
python gcs_viewer.py "stone.gcs" --save out.png --gray --no-labels
python gcs_viewer.py --selftest report.txt # prove a build works
python gcs_viewer.py --version # which build is thisOn Windows use pythonw for the window, so no console appears behind it.
A defaulted output name is never overwritten — save the same design twice and
the second lands beside the first as stone_views (1).png. A name you give
explicitly is used as given.
Those three libraries are the only dependencies — no matplotlib, no scipy. Developed on Python 3.12; CI runs the suite on 3.10, 3.12 and 3.13, on Windows and Linux.
Set GCS_VIEWER_NO_GUI=1 to make errors print to stderr instead of opening a
message box, so an unattended script cannot stall on a dialog.
Tests, the verification tooling and the constraints on changing the parsing API: DEVELOPING.md.
CHANGELOG.md — what changed in each release and why.
MIT — see LICENSE.