Though perhaps best known as inclusions within other gems, rutile crystals themselves can be faceted or cabbed as curiosities for collectors. Rutile can show a deep, red color. Synthetics can show a variety of colors and have even been used as diamond simulants.
Tetragonal. Crystals prismatic, vertically striated, well developed, often twinned into a series of contact twins with up to eight individuals, sometimes looping to form a complete circle! Also, massive; granular.
o = 2.62; e = 2.90. Uniaxial (+). Sometimes anomalously biaxial.
From the Latin rutilus for red, in allusion to the color.
In addition to its occurrence as inclusions in other minerals, occurs as a high-temperature mineral, in gneiss and schist, also in alpine-type veins; found in igneous rocks, pegmatites, regionally metamorphosed rocks, including crystalline limestones, and as detrital grains.
Rutile is polymorphous with anatase and brookite. They share the same chemical formula, TiO2 (titanium dioxide), but have different crystal systems and other gemological properties. These gems are rarely cut, but of the three you’re more likely to encounter faceted or cabbed rutiles.
Rutile also lends its name to the rutile mineral group. The only other gem-quality member of this group that gets faceted (rarely) is cassiterite.
Rutile as Inclusions in Other Gemstones
As needle inclusions, rutile crystals occur inside a wide variety of gem materials, such as quartz and agate (sagenite). Rutilated quartz pieces can make stunning jewelry stones as well as display specimens.
In corundum, rutile crystals occur as fibers, causing asterism in stones such as star sapphires. Rutile inclusions also cause chatoyancy (the “cat’s eye” effect) in gem materials such as chrysoberyl. Sometimes, rutiles themselves may show cat’s eyes when cabbed.
Rutiles have a Mohs hardness value of 6-6.5, which makes them somewhat vulnerable to scratches. Use protective settings for any jewelry use, especially rings. However, one of rutile’s most attractive properties is its exceptional dispersion or “fire.” It will break up white light into many flashing, multi-colored points. At 0.28, its dispersion is six times greater than that of diamond. Jewelry and gemstone enthusiasts usually prize this property.
However, faceted natural rutiles may disappoint collectors because the finished gems are so dark. Rutile’s deep, red color may be so intense it can’t be seen easily in stones larger than one carat. Rutiles with metallic luster will also not show their dispersion well.
Cabochons might show reddish reflections in cracks and along imperfections.
You’re more likely to find natural rutiles in mineral collections than in jewelry collections. However, synthetic rutiles have a history of use as diamond simulants or lookalikes. As jewelry stones, synthetic rutiles may be encountered more often than natural rutiles. They can show greater transparency, which can help highlight their dispersion, and can come in a wide range of colors.
Rutile’s most distinctive characteristic is its high dispersion of 0.28. It surpasses the dispersion of almost all facetable gem materials. Only cinnabar has a higher dispersion. However, it can rarely be cut in a way that shows off this property. (Cinnabar’s other properties will also make it readily distinguishable from rutile).
Rutiles have a color range that overlaps with polymorphs as well as group mates. However, rutiles have greater hardness and specific gravity (SG) values than anatases and brookites. On the other hand, cassiterites are harder and much more dense.
Swiss rutile seems a bit more transparent than natural material from other localities.
Rutile’s SG varies somewhat in relation to its trace elements.
Iron (Fe) bearing: 4.2 – 4.4.
Niobium (Nb) and tantalum (Ta) bearing: 4.2 – 5.6.
Do Synthetic Rutiles Make Good Diamond Simulants?
As a mineral, rutile has many industrial applications. Scientists have synthesized it for research into many areas, including its photocatalytic properties, like those of anatase.
Synthetic rutiles have also found jewelry use, first appearing on the gem market in 1948. Created through the Verneuil or flame fusion process, these transparent, nearly colorless gems with a yellow tinge took full advantage of rutile’s high dispersion.
A 25 x 4 mm disk of synthetic rutile, grown via the Verneuil technique. Photo by Krizu. Licensed under CC By-SA 3.0.
For a time, gem dealers sold synthetic rutiles under the name Titania as diamond simulants. However, these stones showed too much fire to be believable lookalikes. Thus, they faded from use as other simulants emerged, such as cubic zirconia. (Synthetic rutiles are also denser than diamonds).
Synthetic rutiles can also show colors such as yellow, brown, red, and blue. Heat treatments can turn light yellowish synthetic rutiles blue.
Large crystals often have transparent areas that can provide stones for faceting. However, cut rutiles above 2-3 carats are so dark they appear opaque. Thus, this becomes the effective size limit of faceted gems.
Devonian Group (Calgary, Alberta, Canada): 3.70.
Caring for Rutiles
If you do have a synthetic rutile as a diamond simulant, keep in mind that while it may exceed diamond in dispersion, it’s far less hard (6-6.5 versus diamond’s 10). Store it separately from other common jewelry stones such as quartz, topaz, and diamonds to avoid contact scratches.