Radial Limonite in Sapphire
Limonite, a composite of hydrated iron oxide minerals, is an inclusion commonly seen in corundum, usually as a thin yellow veil of epigenetic deposits within surface-reaching fractures. A unique example of this material is shown in the figure above, where limonite precipitated into a small fracture and formed in a radial fashion. When viewed under diffused reflected light, the inclusion displayed bright interference colors. The inclusion stands in sharp contrast to the violet sapphire host. As a composite of hydrated iron oxide minerals, the limonite’s yellow color is a strong indication that the stone has not been heat treated, as this would cause the yellow limonite to alter into red hematite (J.I. Koivula, “Useful visual clue indicating corundum heat treatment,” Fall 2013 G&G, pp. 160–161). This example demonstrates that even the most common of inclusions can occasionally materialize as something remarkable.
