Hematite Inclusion
Hematite Inclusion
The iron-oxide platelets responsible for aventurescence in feldspar gemstones
A hematite inclusion is a discrete crystal or, more characteristically, a thin oriented platelet of hematite (iron(III) oxide, Fe₂O₃) occurring within a host gemstone during primary crystal growth. These inclusions are most consequential in feldspar minerals — particularly the labradorite-andesine series and Oregon sunstone (Heliolite) — where densely packed, co-planar hematite platelets produce the optical phenomenon known as aventurescence: a vivid, metallic flash of red, orange, or copper-gold light that shifts dramatically as the stone is rotated. Hematite inclusions are among the most gemmologically significant of all metallic mineral inclusions, serving simultaneously as a diagnostic fingerprint of origin and as the direct cause of a highly prized visual effect.
Mineralogy and Formation
Hematite crystallises in the trigonal system and, when it forms as inclusions within a feldspar host, typically adopts a platy, pseudo-hexagonal habit. The platelets orient themselves parallel to specific crystallographic planes of the host lattice — a relationship governed by epitaxial compatibility between the two structures. This crystallographic alignment is not incidental; it is the precise reason that all platelets reflect incident light simultaneously and in the same direction, producing a coherent, directional flash rather than a diffuse scatter.
The inclusions form during the slow cooling of magmatic or metamorphic feldspar, as iron that was initially dissolved within the feldspar structure exsolves and migrates to form discrete hematite crystals. The size, number density, and thickness of the resulting platelets are controlled by the rate of cooling and the original iron content of the host crystal. Rapid cooling tends to produce smaller, more numerous platelets; slower cooling allows fewer but larger crystals to develop.
Optical Effects: Aventurescence and Schiller
When hematite platelets are sufficiently numerous and uniformly oriented, they collectively act as an array of tiny mirrors. Incident light striking the stone at the appropriate angle reflects from this array in a concentrated, directional manner — the phenomenon termed aventurescence or, in the older German gemmological literature, Schiller. The colour of the resulting flash is determined by two factors: the inherent reddish-metallic reflectance of hematite itself, and any body colour contributed by the host feldspar.
In Oregon sunstone, hematite platelets yield flashes ranging from copper and orange-red to deep red, depending on platelet density. Stones with the highest concentrations of larger platelets display the most saturated red schiller and are the most commercially desirable. Lightly included stones may show only a faint golden shimmer, while those with intermediate platelet densities produce the characteristic copper flash for which Oregon sunstone is internationally recognised. The GIA has documented that the intensity of aventurescence in Oregon sunstone correlates directly with the concentration and size of hematite platelets visible under magnification.
Occurrence in Named Gem Materials
- Oregon sunstone (Ponderosa Mine and Dust Devil Mine, Lake County, Oregon, USA): The definitive locality for hematite-included feldspar. Oregon sunstone is an andesine-labradorite feldspar whose copper-red aventurescence is attributable entirely to native hematite platelets — a point of considerable commercial importance, as it distinguishes these stones from copper-diffusion-treated feldspars from other sources.
- Aventurine feldspar (oligoclase): The original aventurine feldspar, sometimes called goldstone feldspar in older texts, contains hematite or goethite platelets that produce a golden to reddish schiller. Specimens from Norway and India are the best documented.
- Certain labradorites: Some labradorite specimens display a secondary hematite-related schiller distinct from the interference-based labradorescence for which the species is better known.
Diagnostic Value
In the context of gemstone identification and origin determination, hematite platelets are a primary diagnostic inclusion. The Gübelin and Koivula Photoatlas of Inclusions in Gemstones illustrates their characteristic pseudo-hexagonal outline and metallic reddish-brown colour as seen under reflected light microscopy. Under standard gemological magnification, hematite platelets appear as bright, mirror-like flakes with a distinctly reddish or copper-coloured metallic lustre — quite unlike the silvery appearance of goethite or the brassy tone of pyrite.
Their presence in Oregon sunstone is particularly significant from a trade perspective. Because copper-diffusion treatment has been applied to some feldspar material from Ethiopia and the Democratic Republic of Congo to simulate the aventurescence of Oregon sunstone, gemmological laboratories routinely examine the nature and distribution of metallic platelets to distinguish natural hematite exsolution from artificially introduced copper. Native hematite platelets in Oregon sunstone are randomly distributed in three dimensions throughout the stone, whereas diffusion-related copper tends to concentrate near the surface and along fractures.
In the Trade
Oregon sunstones with strong, evenly distributed hematite aventurescence command significant premiums in the coloured-stone market, particularly in larger sizes where the schiller effect is most dramatic. The GIA issues origin and treatment reports for Oregon sunstone that specifically address the presence and nature of metallic inclusions. Collectors and dealers familiar with the material regard the depth and saturation of the hematite-derived flash as the primary quality criterion, ahead even of body colour transparency in some cases.
It is worth noting that the term aventurine glass (or goldstone) refers to a man-made material containing copper or chromic oxide crystals, and is entirely unrelated to natural hematite-included feldspar, despite a superficially similar appearance to the uninitiated eye.