During recent months, natural rubies with surface cavities filled with glass-like substances that may influence both the appearance and weight of the faceted stone were examined by gemological laboratories in Thailand and England and by the Gemological Institute of America's Gem Trade Laboratories in both New York and Los Angeles. This article reports on the examination and analysis of 15 natural rubies with notable glass filled cavities and simple methods for identifying the presence of such filled areas. In most cases the cavities were readily detected by their distinctive luster and other visual characteristics. X-ray diffraction and chemical analysis indicated that the filler was indeed a glass; it was probably added in conjunction with heat treatment. -- AATA