Fracture filling injects colorless substances—lead glass, resin, or wax—into surface-reaching breaks to improve apparent transparency and stability. In rubies and sapphires, lead-glass filling can dramatically enhance clarity but lowers durability because the filler may be damaged by heat, chemicals, or ultrasonic cleaning.
Detection shows flash effects (colorful reflections), gas bubbles, or flow lines under magnification, plus lower specific gravity or unusual fluorescence. GIA and other labs grade the extent of filling. Heavily filled stones receive significant price discounts due to reduced longevity and the need for special care.