At the Bench

Grown, not found · Field guide

Workshop

What each material actually does under the lap. Hardness is only part of the story — cleavage, heat tolerance, orientation and refractive index all decide whether a piece of rough becomes a stone or a lesson.

The four things that decide how a material behaves

1. Hardness sets your lap sequence and how long it takes. It does not tell you how tough the material is. Diamond is the hardest thing there is and cleaves perfectly in four directions.
2. Cleavage is where the material would rather split than be cut. Distinct or perfect cleavage means light pressure, careful transfers, and no shortcuts.
3. Heat tolerance decides your dopping method. Anything strained (Verneuil boules), anything glassy (nanosital), anything that conducts heat poorly (CZ), and anything with fluid inclusions gets warmed and cooled slowly, or dopped cold.
4. Refractive index decides the design. Higher RI, shallower pavilion.

Material by material

Critical angles, for design selection

Pick a design specified for the material's refractive index. If you must adapt one, this is the number that governs whether the pavilion returns light or leaks it.

General notes that apply to most grown material

• Verneuil rough is strained. Boules split lengthwise on cooling for a reason. Preform gently, don't shock it thermally, and expect the occasional piece to come apart.
• Check for a seed plate in anything hydrothermal before you preform. Plan the stone so it falls outside the finished shape, or somewhere it won't show face-up.
• Cubic materials have no orientation. Spinel, CZ, YAG, LuAG, GGG, GAGG, fluorite, diamond. Preform however the rough suits you. This is a real time saving.
• Uniaxial materials generally want the table perpendicular to the c-axis — that puts the optic axis vertical, which minimizes visible doubling and usually gives the best face-up color. Corundum, beryl, quartz, moissanite, rutile.
• Color-change material is oriented for the change, not for brightness. Preform, then look at it under both light sources along different axes before you commit to a table.
• Overpolishing is a soft-material problem. Below about hardness 7, dwelling on the polish lap rounds facet edges and blurs meets. Light pressure, short passes, check often.
• Grown material is usually inclusion-free, which means yield is high and you can plan the stone to fill the rough. That's a genuine advantage over natural material and it's part of what makes synthetics good for learning.

Polish, roughly

Every cutter has their own combination and laps vary enormously, so treat this as a starting point rather than doctrine.

A reference to lab-grown gem materials, written for cutters. Values are typical published figures; ranges vary with composition, dopant and measurement wavelength. Nothing here is a substitute for a laboratory report on a stone whose identity matters commercially.