Let's Talk Gemstones

By Edna B. Anthony, Gemologist

Staurolite

AL2SiO5 GROUP / A NESOSILICATE

Staurolite! Why would one of the most ordinary, patently unattractive minerals, used as a religious talisman and a good luck charm for centuries, be, in its transparent faceted form, a much sought after collector's gem? Rarity! Seldom do the dark reddish-brown or yellowish-brown transparent crystals occur in sufficiently light tones and sizes to warrant faceting. Its fair dispersion of 0.023 is usually masked by its excessively dark tones. A sparkling two-carat staurolite gem would certainly be a true prize.

Idiochromatic staurolite develops in regional metamorphic rock formations. In petrology, it is used as an index mineral to define the grade and type of the host rock. Staurolite rarely occurs in massive form. It occurs more often in opaque, interpenetrant, twinned forms at sixty degree, as in St. Andrew's cross, or ninety degree, as the Greek cross, orientations than in individual prismatic crystals. The cruciform shape gives it its name derived from the Greek words stauros (cross) and lithos (stone). Layers of chains of AlO8 octahedra parallel to the C axis alternate with layers of Fe2AlO3(OH) in its nesosilicate structure. There is normally the replacement of some of the Fe2+ by magnesium, with Fe3+ replacing a portion of the aluminum. Dr. Frederick H. Pough states in his A Field Guide to Rocks and Minerals that "it can be regarded chemically as a mixture of kyanite and iron hydroxide." Although it is slightly affected by sulfuric acid, the mineral is infusible and virtually insoluble in acids. After firing, it crushes to a slightly magnetic brown powder. The streak of fresh material is colorless.

Both the Color Encyclopedia of Gemstones by Dr. Joel Arem and the Manual of Mineralogy list its crystal system as monoclinic (pseudo-orthorhombic), but other sources classify it as orthorhombic. A distinct plane of cleavage exists, and the fracture of this brittle substance is conchoidal to uneven. A density range from 3.65 to 3.83 with a hardness of 7 - 7.5 is typical. The lustre and fracture lustre can vary from vitreous to resinous to dull, and a pocked surface may appear if a degree of alteration has occurred. Its normal refractive indices [a=1.739-1.747; b=1.745-1.753; y=1.752-1.761] increase with the iron content. The birefringence of this biaxial positive mineral has a range of 0.011 to 0.015. Golden yellow/red or yellow/colorless are its distinct pleochroism colors. A strong band at 4490 and a weak one at 5780 occur in the non-diagnostic spectrum. Exposure to long-wave and short-wave ultraviolet light causes no reaction.

With the presence of a trace of zinc, lighter colors occur in zincian staurolite, and some of its optical and physical properties differ slightly. A specific gravity of 3.79 is normal. Its refractive indices range from 1.721 - 1.731. Beyond 4900, the spectrum is absorbed, but weak narrow bands occur at 5315 with wide strong bands exhibited at 6100 and 6320. Yellow/green/red are its trichroic pleochroism colors. Zincian staurolite might even be considered a color-change gem since it may appear yellow-green in daylight and red-brown in incandescent light. Generally, it is from this material from Switzerland or Brazil that the rare gemstones are cut.

Rock-forming deposits of strongly pleochroic lusakite, a deep blue opaque staurolite, where cobalt and magnesium replace iron, are located in Lusaka, Zambia. The lusakite is extracted from the formations and used as a pigment.

Staurolite has been known by many names. Mineralogists and geologists will be familiar with staurotide and grenatite. Fairy stones, fairy crosses, and cross stones are abundant in North Carolina, Virginia, Georgia, and Tennessee. They are worn as amulets and good luck charms. Imitations are carved from fine grained clay rock and dyed. A source near Taos, New Mexico produces fine twinned staurolite. Deposits in Connecticut, Maine, Vermont, New Hampshire, Canada, Scotland, France, some alluvials in the Urals and the Gorob mine in Namibia are other sources. Staurolite found in Sweden contains a significant amount of manganese. Kyanite accompanies the twinned staurolite recovered from the mica-schists of Monte Campione and Lake Ritone in Switzerland. Names used here are Lapis crucifix and Baseler Taufstein (Basle baptismal stone). Columnar crystals of staurolite also occur with kyanite in these white schists and are striking mineral specimens.

Collectors of mineral specimens usually have staurolite specimens in their cabinets. However, a fair-sized clean light-colored faceted staurolite gem is a real treasure that few possess.


 
Gemstone Properties
SPECIE
staurolite
Composition:
[FeMg]2[AlFe] 9 O 6 [SiO4) 4 [O,OH]2+Zn or +Co Hydrous iron-aluminum silicate
Class:
silicate
Group
Al 2 SiO 5
Species:
staurolite
Crystal System:
monoclinic; pseudo-orthorhombic; orthorhombic
Variety:
staurolite; zincian staurolite
Colors:
dark brown, reddish brown, dark blue, and black
Phenomena:
color-change? see narrative
Streak:
colorless or light gray
Diaphaneity:
opaque, translucent, and rarely transparent
Habit:
columnar, prismatic, typically twinned, and rarely massive
Cleavage:
distinct in one plane
Fracture:
conchoidal to uneven
Fracture Lustre:
resinous
Lustre:
vitreous to resinous to dull to earthy 
Specific Gravity
3.65 to 3.83
Hardness
7 to 7.5
Toughness:
fair
Refractive Index
1.721 to 1.761; see narrative
Birefringence:
0.011 to 0.015
Optic Character
biaxial positive
Dispersion:
0.023
Pleochroism
distinct trichroic; less red or yellow/golden yellow; zincian staurolite: yellow, green, red
Luminescence
inert
Absorption Spectrum
see narrative
Aqua Filter
no information
Chelsea Filter
no information
Solubility
affected slightly by sulfuric acid; virtually insoluble in acids
Thermal Traits
infusible
Treatments
oiling of cross stones
Inclusions
no information