A post-World War II development of the National Lead Co. is (and now produced also by Linde Air Products Co.) was the synthesis of titanium dioxide, or rutile. In nature rutile is an important mineral but one that has very seldom, if ever, been used for gem purposes by jewelers. Usually, this is a black, opaque mineral with no gem possibilities; however, on occasion, it is found in a dark-red color that lends itself to use in a limited way. In gem quality, synthetic rutile is most commonly light yellow in color, because it is almost impossible to produce it without being slightly contaminated by iron. The deliberate addition of iron results in orange to orangey-brown colors. These colors, however, can be altered by heat treatment, producing attractive blue and bluish-green stones.
Despite its rather low hardness (about 6), synthetic rutile has interest as a gem material, because it has an enormous dispersion (approximately 0.330, compared to diamond's 0.044), and is considerably higher in refractive index. Notwithstanding erroneous claims that the R.I. makes it more brilliant than diamond, it is actually considerably less brilliant. It is slightly less transparent, does not take the same optically flat polish that is characteristic of diamond, and much of the white light that enters it is broken into dispersed beams or divided by the material's enormous birefringence into double beams of correspondingly lower intensity, or brightness. In other words, whereas light leaving diamond emerges as clear, brilliant flashes, light leaving synthetic rutile is altered both by the birefringence and the dispersion.
Furthermore, the lower degree of transparency causes more of the light that enters it to be absorbed. A refractive index higher than diamond in a transparent material does NOT prove that it has greater brilliancy.
Two very important factors are related to R.I. First, the higher the
R.I. the more light that reaches a polished transparent material is
reflected from the first surface and not refracted into the stone; thus,
there is less light available for total reflection at the back facets.
An extreme illustration of this is proved by the metals, which have such
high refractive indices that most of the light striking the surface,
even at right angles, is directly reflected. The second factor operating
in inverse relationship to the R.I. is the size of the critical angle:
the higher the index, the smaller the angle. This is fortunate, when we
recall the picture of total reflection as it operates within a stone at
the back facets. But at the inner surface of the crown facets, where
light should leak out to the eye, the small critical angle provides such
a small "target" that much light is again totally reflected internally,
finally finding its way out of the stone at perhaps some less desirable
area, in so far as contributing to the brilliancy of the stone is
concerned. For example, a material with an index of 4.0 would have a
critical angle of approximately 14°.
Synthetic rutile was first announced in 1948, and for a period of several years thereafter it enjoyed great popularity (for, at least, notoriety). It was marketed by many firms under almost as many names, including some of the following :
|Astryl||Rainbow Magic Diamond|
|Johannes Gem||Tiru Gem|
|Kima Gem||Titannia Brilliante|
|Kimberlite Gem||Titania Brilliante|
|Lusterite||Titania Midnight Stone|
|Rainbow Diamond||Titaniium Rutile|
Advertisements in every kind of magazine, from the "pulps" to the slick-paper types, proclaimed its brilliancy as superior to diamond and implied that in every other respect it was also superior. Much of the material was sold at fairly high prices per carat. Since it was sold principally for mounting as a ringstone, a use for which it is not well adapted, it soon developed an unfortunate reputation, which could have been avoided had it been used in the kinds of jewelry pieces that receive less abuse. Its use as a ringstone is ill-advised, for the rather low hardness and durability mean that it is almost certain to suffer damage on the hand of even a careful person. As a result of the exaggeration of its quality and the poor choice of uses for a stone of such low durability, this unfavorable reputation exists virtually undiminished today. Few jewelers are willing to handle synthetic rutile, for fear that it will be returned damaged in a few weeks or months by unhappy and dissatisfied customers.
The chemical formula of synthetic rutile is TiO2, and it crystallizes in the tetragonal system in boules similar to those of synthetic corundum. The specific gravity is 4.24 to 4.26. The hardness, as mentioned previously, is approximately 6; it is usually given by the manufacturers as 6 to 6-1/2, but it is relatively easily scratched and fractured by a 6 hardness point. The refractive indices are 2.616 to 2.903. Since it is uniaxial positive, the value for the ordinary ray is 2.616 and that for the extraordinary ray is 2.903; this means that the index parallel to the C-axis is the lower of the two. The melting point is 1825°C. It has a vitreous luster; a poor to fair degree of toughness (damage during setting is rather frequent); poorly developed prismatic and pyramidal cleavage; a conchoidal to uneven fracture; and a birefringence of 0.287. It is resistant to most acids and alkalis, but excessive heat may cause a change in color.
The stone can be identified readily by its enormous birefringence and tremendous dispersions; the latter, however, is not as pronounced in the colored specimens as in the light yellows. As with any doubly-refractive material, the orientation may be such that maximum birefringence, or doubling, may not be visible through the table. Figure 3 shows extreme doubling of back facet junctions and the culet, and Figure 4 illustrates how the typical gas bubbles of the flame-fusion process appear in pairs as a result of the excessive birefringence.
The only stones with which it might possibly be confused are zircon
and sphene. Zircon has a much lower birefringence, is harder than
rutile, and, in a light color, has a much higher S.G. Sphene, too, has a
very high R.I., but is lower in S.G. (3.52) and has less than half of
the birefringence; also, it is biaxial, rather than uniaxial.
Thus, synthetic rutile is rather easily identified.