Nail-head spicules are inclusions that have traditionally been associated with hydrothermal synthetic quartz and emerald. They are caused primarily by rapid growth conditions and disturbances during crystallization of their host. However, these or similar-looking inclusions have also been found in natural gems. The authors examined a natural emerald and blue sapphire showing true nail-head spicules, and a yellow sapphire, spinel, diamond, and rock crystal quartz with inclusions that strongly resembled them. Nail-head spicules remain a notable feature of rapid and disturbed growth, but they do not confirm a stone’s natural or synthetic origin without further examination.
