Anatase occurs in many beautiful colors, such as deep indigo and amber yellow. However, these rare gems are seldom transparent and are usually found as very small crystals. They’re hardly ever faceted, except as curiosities for gem collections.
o = 2.534-2.564; e = 2.488-2.497. Uniaxial (-). Some crystals may be biaxial. See "Identifying Characteristics" below.
Uniaxial -, Biaxial -
From the Greek anatasis for “extension,” since the anatase “octahedron” is really a tetragonal bipyramid and, thus, elongated with respect to the octahedron of the isometric system. Octahedrite alludes to this “double pyramid” shape. Oisanite references the type locality, St Christophe-en-Oisans, France.
Gneisses, schists, and other metamorphic rocks; as detrital grains, as an accessory mineral in granites, and in various other igneous rocks.
Anatase crystals, even when transparent, tend to have darker colors. This masks the gem’s high dispersion, which exceeds that of diamond. Very dark faceted gems may appear unattractive.
You might find references to anatase as “octahedrite.” The name comes from the “double pyramid” shape typical of many anatase crystals. However, despite this occasional appellation, anatases don’t form true octahedrons nor belong in the isometric crystal system.
Xanthitane is a soft, friable (crumbly), yellow-colored sphene (titanite) pseudomorph of anatase. Pseudomorphs are minerals that change chemistry without changing their crystal form. In this case, from TiO2 to CaTiSiO5.
Transparent, faceted gems may show birefringent doubling.
Of the three polymorphous titanium dioxide gems, anatase is the rarest.
You may encounter faceted rutile, especially synthetic rutile, more commonly than faceted anatase. The deeper color ranges of these two gems overlap, and they actually both have tetragonal crystal structures. However, rutile has a higher specific gravity (SG) and greater hardness. Rutile’s optical properties also differ. For example, it has a uniaxial (+) optic character.
Rare faceted brookites have some color overlap with anatases (but rarely blue) and share the same hardness. However, orthorhombic brookite has different optical properties, including a higher birefringence. It also has a higher SG.
Some more popular gemstones that share the same hardness and color range of anatases include transparent scapolites and opaque sodalites (blue). However, both these gems have luminescent properties, unlike anatases, as well as other distinguishing characteristics.
Before the official adoption of the name “anatase,” gems such as this were called oisanites, after the type locality, St Christophe-en-Oisans, Bourg d’Oisans, Isère, Rhône-Alpes, France. This beautiful topotype on matrix comes from the same deposit. Photo by Didier Descouens. Licensed under CC By-SA 4.0.
Anatase can act as a photocatalyst. For example, if exposed to light while underwater, anatase absorbs photons, which excites electrons in its atomic structure. In turn, these electrons split the water molecules into hydrogen and oxygen. Since such a reaction has many possible applications, such as disinfecting water, cleaning oil spills, and creating solar cells, scientists have synthesized anatase, including crystal forms, for research purposes. However, there is no known jewelry use for this synthetic material.
You’ll more likely find anatase in a mineral collection than a jewelry collection. Due to this gem’s perfect cleavage and hardness of 5.5 to 6, protective settings and occasional wear would be advisable. Lighter, transparent gems could display anatase’s dispersion and birefringence, while darker, opaque material might showcase its adamantine to metallic luster. Store anatases separately from other gems to avoid contact scratches. Clean them with a soft brush, mild detergent, and warm water. For more care recommendations, consult our gemstone jewelry cleaning guide.