The
principal producing opal mines of Mexico are at present in the State of
Queretaro, though fine stones have come from Zimapan, Hidalgo. The mine
examined is near the top of a range of hills composed of reddish-gray
spherulitic rhyolite. The mine was opened in this rock some six years
ago at a point where opals had been exposed by weathering. The opal is
obtained by quarrying and later breaking up the blocks into 2-inch
cubes or less. The face of the quarry is about 40 feet high of which
the lower 25 feet is productive. The upper 15 feet of rhyolite is
decomposed and carries no opals. It is stripped to the opal-bearing
layer and the latter is quarried. Gunpowder is used in blasting the
barren rhyolite, but dynamite is necessary in the hard opal-bearing
rock. The limit of depth to which opal will be found is not known, but
other mines are reported to be producing opal at a depth of 100 feet.
Surface
indications for opal called "pintas" consist of a white siliceous
sinter filling cavities in the rhyolite. These are considered
infallible but furnish no information as to the quality or quantity of
opal to be expected. A corroded appearance on the working faces and
numerous cavities containing deposits of silica are common in the
quarry. Many of the cavities are lined with clear brittle quartz and
contain loose deposits of opal. In other cases opal fills the entire
spaces in which it occurs, and is intimately joined to the rock either
with or without surrounding iron stain. Opal of good quality is found
in both occurrences.
About
once a week the rough opal is taken to Queretaro for polishing. The
stones are first roughly shaped on ordinary large grindstones into the
largest sizes consistent with symmetry and beauty. The roughly cut gems
are then smoothed on a sandpapering machine and given a final polish on
soft rough leather.
QUARTZ. OHIO.
Occasional
clear colorless pebbles are found in the gravel beds in the vicinity of
Windsor, Ohio, and a few have been cut for gems. Several small rough
specimens and a cut gem were received from Mr. S. Stoughton, of
Windsor, for identification. They proved to be transparent quartz
pebbles of sufficiently good quality for cutting where such material is
desired.
ROSE QUARTZ.
VARIOUS LOCALITIES.
Rose
quartz occurs at many localities in the United States, and varying
quantities from a number of these have been cut for gems and ornamental
purposes. The principal output has come from the Red Rose mine of
Samuel Scott, 6-1/2 miles S. 50° E. of Custer, S. Dak. This mine was
described in this report for 1908. Other deposits of rose quartz occur
in the Black Hills of South Dakota, but so far none have yielded good
dark gem material. Rose quartz has been found at several places in
Colorado, and promising material occurs in the Wild Rose claim of J. D.
Endicott, 6 miles north of Texas Creek, Colo. Deep rich pink rose
quartz has been found in Maine, but much
