Raw ruby is a variety of the mineral corundum, a material with exceptional strength and durability that makes it a highly desired gemstone. It ranks 9 on the Mohs scale of mineral hardness, which determines scratch resistance. Because corundum is extremely hard, traditional abrasive materials are ineffective for shaping and polishing. The process requires specialized tools and the use of diamond, the only natural material harder than a ruby. Transforming a rough, raw crystal into a brilliant, polished gem relies entirely on diamond abrasives to achieve the final luster.
Preparation and Necessary Tools
Successfully polishing a raw ruby requires a dedicated lapidary machine to handle the stone’s hardness. Home hobbyists typically use a cabochon grinding and polishing machine (cabbing machine) or a flat lap machine, both featuring interchangeable diamond-coated wheels or discs. Cabbing machines are generally preferred for shaping curved surfaces, such as cabochons, and include a sequence of wheels with different grit sizes for a seamless transition.
Diamond abrasives are the only compounds capable of cutting and polishing corundum, available as grinding wheels, pads, or paste. Initial shaping requires coarse diamond wheels, typically 60 to 220 grit. Subsequent smoothing and final polishing require a progression of finer diamond pads or pastes, with grits ranging from a few hundred up to 50,000 or higher.
The process must be performed wet, requiring a reliable water cooling system to prevent the stone and tools from overheating. Overheating can cause thermal shock, potentially damaging the ruby or weakening the bond on the diamond tools. Safety equipment is equally important for protection against fine, airborne particles. Proper eye protection, a dust mask, and adequate workshop ventilation are mandatory before starting any work.
Shaping the Rough Stone
The shaping stage involves aggressive material removal to establish the stone’s final geometry, whether a rounded cabochon or a faceted pre-form. This initial grinding uses the coarsest diamond abrasives (typically 60 to 220 grit) to quickly remove inclusions, fractures, and unwanted material from the raw crystal. The goal is to define the exact shape and proportions, focusing on establishing smooth curves or precise angles.
Apply consistent pressure while holding the stone against the rotating coarse diamond wheel, continuously moving the ruby to prevent flat spots or grooves. This stage generates significant heat, so a constant flow of water coolant is essential to flush away the grinding slurry and maintain an optimal working temperature. Shaping is complete when the desired form is realized, and the surface exhibits a uniform texture of deep scratches left by the coarsest abrasive.
Smoothing and Refining the Surface
Once shaped, the focus shifts to scratch removal using a precise, progressive sequence of finer diamond grits. This intermediate stage is the most time-consuming, involving the elimination of deep scratches left by initial shaping. A typical progression involves moving through grits like 400, 600, 1200, and 3000, ensuring all marks from the previous stage are completely erased before advancing.
The lapidary must work patiently on each grit level, as skipping a stage or failing to remove prior scratches will leave permanent flaws that become more visible in the final polish. Water cooling remains necessary, but pressure should be lighter than during shaping to control the material removal rate. Between each grit stage, the ruby must be thoroughly cleaned to prevent contamination. Contamination occurs if a single coarse diamond particle transfers to a finer wheel, causing deep, new scratches. Visual inspection with magnification is required after each stage to confirm the surface texture is uniformly covered only by the markings of the current abrasive.
Achieving the Final Luster
The final stage achieves the high-gloss, mirror-like finish that maximizes the ruby’s light return and color saturation. This requires ultra-fine diamond compounds, typically paste or powder, ranging from 8,000 to 50,000 grit or finer. These microscopic particles are applied to a specialized polishing surface, such as a tin, ceramic, or felt lap, which holds the compound for the final friction polish.
The technique for the final polish differs significantly from grinding, requiring very light pressure and a slower machine speed. The goal is burnishing, which creates a microscopically smooth surface capable of perfectly reflecting light, rather than material removal. The polishing compound, often mixed with a lubricant like water or oil, forms a slurry on the lap surface that gently smooths the stone. The process is complete when the ruby exhibits a flawless, reflective surface with no visible haze or fine scratches.