Glass should be considered safe for body piercing jewelry, and I don’t think this is a matter of opinion. The science supporting the use for glass is well established. From protective encapsulantion for electronic implants and RFID tags to tissue cultures and skin grafts contained in petri dishes, glass has been proven virtually non-reactive to human tissue. In the revised Minimum Standards for Jewelry for Initial Piercing,
The APP’s Minimum Standards for Jewelry for Initial Piercings is considered by many to be the definitive statement on the subject of proper jewelry materials and quality for healing piercings. As this document is increasingly integrated—in some form or another—in legislation and written policy in the United States and abroad, the importance of keeping it current becomes glaringly important. And while the APP’s focus is not creating and enforcing jewelry standards (even though there are certainly those who feel it should be) it is important the organization’s standards for membership are current, and reflect the collective knowledge of not only the membership, but of the industry as a whole. — James Weber, APP President, The Point: The Journal of the Association of Professional Piercers, Issue 46
“Fused quartz glass, lead-free borosilicate or lead-free soda-lime glass,” was accepted as a material for body piercing by the Association of Professional Piercers after much research and experiment.
One of the ASTM committees that I participate with is F04.15 on Material Test Methods, and one of the standards under our jurisdiction is F561 which covers “Standard Practice for Retrieval and Analysis of Medical Devices, and Associated Tissues and Fluids” with an excellent section describing glass and ceramic materials used in surgical implant.
“X7. RATIONALE X7.1
The ultimate test for an implant or device and the materials out of which is fabricated, is its performance as an implant. In many situations, critical information can only be ascertained by examination of retrieved implants and the surrounding tissues. Such analysis could be part of experimental studies in animals, clinical studies, where devices are routinely removed or removed for revision, or in post-mortem autopsy studies. The information suggested for collection could also be of value in device tracking.”— ASTM F561
Glass removed from surgical implants, or body piercings, has not been shown to be the cause of chemical or surface finish problems, only issues related to friction wear or accidental damage. From a historical perspective, glass has been used since early human era, and frequently over the last century in the human body. Glass was proven biocompatible by medical studies over a century ago, and used in surgical implants documented as early as 1925 and before.
“A surgeon in Boston, Massachusetts, Dr Marius N Smith-Petersen, MD, introduced the mould arthroplasty (1925). He used  a reactive synovial like membrane that he found around a piece of glass in a workman’s backyard. The original design was ball-shaped hollow hemisphere of glass as shown in Figure 2, which could fit over the ball of the hip joint. The objective was to stimulate cartilage regeneration on both sides of the moulded glass joint. Smith-Peterson intended to remove the glass after the cartilage had been restored. Glass provide new smooth surface for movement. While proving biocompatible, the glass could not withstand the stresses of walking and quickly failed.”— Chronology of Total Hip Joint Replacement and Materials Development
Other articles on glass as body piercing jewelry
As you will note in a review of the literature, glass is considered inert and safe for the body, with the obvious exception of load bearing properties.