Facet

Content sourced from Wikipedia, licensed under CC BY-SA 3.0.

Facets are flat faces on geometric shapes. In crystals and gemstones, facets show the crystal’s symmetry and help reflect light to make the gem look brighter.

Faceting history and cuts
- Early diamond work simply polished the rough stone. Faceting, cutting many flat surfaces, started in Europe in the 14th century.
- The round brilliant cut is the most famous facet pattern for diamonds and many colored stones. An early version is credited to an Italian named Peruzzi in the late 1600s.
- In 1919, Marcel Tolkowsky calculated what were called “ideal” angles for the best light performance. Modern cuts are still guided by his ideas.
- Other historic cuts include the Old European brilliant (an earlier round cut with larger flaws), the Old Mine Cut (rectangular), and the Rose Cut (a flat back with a faceted dome).
- Some stones include a small bottom facet called a culet to prevent chipping; older cuts often had larger culets.

What faceting achieves
- The goal is to balance brilliance (white light), fire (colorful dispersion), and scintillation (sparkling flashes).
- Most faceted stones are transparent to translucent, though some opaque stones like pleonaste or black diamonds are also cut for their luster.

Light behavior and gem quality
- The way light reflects inside a gem depends on the refractive index of the material. Higher indices usually give more internal reflections and more brilliance.
- If light exits the gem instead of reflecting inside, the stone can look dull. This is called light leakage or windowing, common in poorly cut stones.

How faceting is done
- A faceting machine uses a flat disk called a lap and hard diamond abrasives. Water is used for cutting; oil or water for polishing.
- The stone is mounted on a dop stick and held by a part of the machine called a quill. The machine’s angle readout and gear help set precise facet angles.
- The process is repeated: cut each facet to the proper angle, then polish it. A transfer jig lets jewelers flip the stone to facet the other side.
- Historically, before modern machines, wooden dop sticks and simple mounting systems (jamb peg machines) were used, requiring great skill.

Other methods and natural facets
- Some cutting methods use curved, concave facets produced with special tools.
- Crystals often grow with natural facets along low-energy planes defined by their crystal structure. The Bravais lattice points to these planes, and the overall shape can be predicted by concepts like the Wulff construction. Growth conditions, the surface where a crystal sits, temperature, humidity, and pressure all influence the final shape.