The quality of a rhinestone is primarily determined by its material composition, cut precision, and coating. High-index lead-free crystal offers the best refraction. You should look for stones with multi-faceted cuts (e.g., 12 to 16 facets) as they reflect light more intensely. Ensure the back plating (foiling) is smooth and uniform; a high-quality silver or gold vacuum plating prevents oxidation and enhances sparkle. For bulk orders, request a refractive index report to ensure the stones meet your brilliance requirements.