corundum, fashioning.9, synthetic corundum.7, gems

Eppler examines crack-like surface markings on polished corundum, commonly called fire marks or chatter marks, and challenges their widespread use as evidence of synthetic origin. Although frequently observed on synthetic ruby and sapphire, these features also occur on natural corundum and therefore cannot independently distinguish natural from laboratory-grown stones. The article reinforces Gübelin’s earlier warning that such markings reflect polishing conditions, while noting that this qualification had received insufficient attention in gemmological literature and teaching. Photomicrographs compare markings on synthetic sapphire with examples on natural sapphires of different colours and geographical origins, including sapphires from Ceylon and a greenish-blue Australian specimen. A rose-coloured Ceylon sapphire displays both surface fire marks and a healing fissure composed of small liquid inclusions. Images of chatter marks on turned steel illustrate the resemblance responsible for the alternative terminology. In metalworking, these patterns arise through displacement and overlapping of material associated with vibration, excessive loading, unsuitable turning speeds, or other operating conditions; the comparison establishes a morphological resemblance rather than an identical mechanism of formation. Eppler emphasizes the parallel arrangement of the corundum markings and their corresponding segments, together with characteristic angular relationships. Some examples show angles approaching ninety degrees, whereas others approach 120 degrees. Their appearance ranges from broad marks to fine fissure-like lines. A higher-magnification view of an Australian sapphire shows a marking continuing beneath the polished surface, supporting its interpretation as a wedge-shaped crack rather than a purely superficial blemish. Accepting excessively rapid polishing or localized overheating as the initiating cause, Eppler proposes that localized stress produces parting along planes of the primitive rhombohedron. The orientation of each polished facet relative to these crystallographic planes would then determine the directions, shapes, and angles of the resulting fissures, accounting for their varied appearance. He extends the same explanation to comparable markings on natural ruby. The article’s principal contribution is an illustrated correction to a misleading identification criterion: fire marks are polishing-related features of corundum that can occur in both natural and synthetic material, and their presence must not be treated as conclusive evidence of synthetic manufacture.