_Composition_: Be₂FeY₂Si₂O₁₀. (Various other rare-earth elements may
substitute into this mineral structure.) _Crystal system_: monoclinic.
_Hardness_: 6.5 to 7.0. _Specific gravity_: about 4.2. _Luster_:
vitreous to greasy. _Color_: black; in thin splinters dark bottle
green. _Streak_: white to greenish. _Cleavage_: none. _Fracture_:
conchoidal to splintery. _Tenacity_: brittle. _Diaphaneity_: opaque to
subtransparent in thin pieces. _Refractive index_: variable, about
1.77 to 1.82.
Gadolinite as a cut gem is not seen outside of large collections;
however, it can be faceted into black opaque stones of little beauty but
of great interest to collectors. The best known locality of this mineral
in the United States is Baringer Hill, Llano County, Texas.
Unfortunately, this locality was completely flooded by the completion of
Buchanan Dam in 1938. Masses and rough crystals of gadolinite weighing
over 100 pounds were mined from this locality. The gadolinite occurred
in a large, very coarse-grained pegmatite dike associated with quartz,
microcline, and fluorite, as well as allanite, fergusonite, nivenite,
cyrtolite, thorogummite, and various other rare minerals. Some of the
minerals in the dike occurred in very large masses. One quartz mass over
40 feet in diameter was noted, and microcline masses up to 30 feet in
diameter were not uncommon. Much of the gadolinite was used by
industrial firms as a source of thorium compounds, although some
specimen and gem material found its way into museums and private
collections. Because the locality was worked mostly from 1910 to about
1925 and because since 1938 the waters of Lake Buchanan have completely
flooded the entire area, material from this locality is now exceedingly
difficult to obtain. The collection of the Smithsonian Institution,
Washington, D.C., contains a cut and polished gem of Baringer Hill
gadolinite that weighs 8.6 carats. This mineral is radioactive because
of the presence of uranium, thorium, and other rare radioactive
elements.