|Transparency||Transparent to opaque|
|Fracture||Conchoidal to splintery|
|Hardness||5 to 5.5|
|Optical Character||Biaxial + Double Refractive|
|Refractive index||1.900 - 2.034|
|Birefringence||0.1 to 0.134|
|Pleochroism||Distinct in brown stones; greenish-yellow, reddish-yellow & colorless.|
|Comments||Attacked by acids|
Sphene (pronounced SFEEN) is a gemstone that is somewhat more frequently used than most of the others discussed under the heading of rare gemstones and ornamental materials in this course. It is a calcium-titanium silicate, corresponding to the formula CaTiSiO5. The name comes from the Greek word meaning "wedge", because of the flattened wedge-shaped monoclinic crystals in which it occurs. The crystals may also be twinned. Titanite, the alternate name used by mineralogists, was given to it because of the element titanium in its formula. It varies from transparent to opaque and may be green, yellow, brown, or combinations of two or all three colors. The very finest is a magnificent green resembling fine emerald, but it is so rare that only a few specimens are know to exist. The usual stone is brownish yellow or yellow-brown.
Sources of sphene include the Tyrol and Salzberg areas in Austria; the St. Gothard district in Switzerland; France; Madagascar; Russia; Pennsylvania, Maine, Canada and, recently, the Baja California area of Mexico. The latter deposit illustrates the nature of many of the gem producing primary deposits of the world. In the late 1940's, a number of large, transparent stones were found by a Mexican prospector.
The discovery came to the attention of a colored-stone dealer, who, with the assistance of the prospector, quickly mined out the entire deposit. Thus, a few hundred carats of sphenes were suddenly available on the market, after which the supply was cut off. A few years later, a second deposit, producing less transparent material, was found in the same general area, but it was unimportant compared with the first. When small deposits of this kind are discovered, it can change the nature of the market for such rare stones very quickly, but only temporarily. Henceforth, textbooks will list Mexico as a source of sphene, but it is possible that no additional material will be produced in the same area.
Sphene occurs in metamorphic rocks, especially schists and gneisses, as well as in certain limestones. It is a typical contact- metamorphic mineral. Associated minerals are zircon, scapolite and apatite. It may also be found with moonstone and smoky quartz in cavities in gneiss.
The luster of sphene is adamantine, its hardness is 5 to 5 1/2, its toughness is only fair (because of distinct, easy cleavage), it has a conchoidal to splintery fracture, and its S.G. is 3.50 to 3.54. The usual R.I. is 1.900 - 2.034; thus, its birefringence of .134 is high. The optic character is biaxial positive. The usual yellow-to-brown stones show distinct greenish-yellow, reddish-yellow and colorless pleochroism. The dispersion is .051, which is greater than that of diamond; this is in accord with all transparent minerals that contain titanium. It is attacked by acids, is fusible, and exhibits no phenomenon. No inclusions have been noted that can be considered characteristic.
The very strong birefringence and noticeable fire of sphene may cause it to be confused with zircon or possibly synthetic rutile; however, in contrast to both stones, it is biaxial in character, rather than uninxial. In addition, its S.G. is much lower than either of these minerals.
Sphene is usually fashioned in the brilliant-cut style, although the emerald cut is sometimes used. Little difficulty is encountered, except that the facet edges have a tendency to crumble. Polishing on a tin lap with Linde A, using angles of 35° for the crown and 41° for the pavilion, prove satisfactory for this highly refractive and dispersive material, When finely cut and polished, it is truly a striking gemstone.
Perhaps the finest exhibit of cut sphenes in America is a series of sixteen matched stones for a necklace, the largest weighing more than fifteen carats, in the U.S. National Museum in Washington, D.C.