Imitations and Reconstruction 213
breathed upon, acquire a thicker coating of moisture than glass and lose it more quickly.
In the advance chapter from the Mineral Resources of the United States—calendar
year 1906 of the United States Geological Survey, entitled, " The
Production of Precious Stones in 1906," by Douglas B. Sterrett, appears
a valuable suggestion for a simple method of testing some precious
stones, made by Mr. M. D. Rothschild, a recognised authority on gem
minerals (this extract was republished from The Jeweller's Circular-Weekly, of January 16, 1907) :
"
The test is applicable to a number of minerals, and can be made by any
jeweller who will exercise care. Hydrofluoric acid or " white acid " (a
mixture of ammonia and hydrofluoric acid) is used. The acid should
never be allowed to come into contact with the skin, as it is very
poisonous and highly corrosive, producing painful sores and ulcers.
The stone to be tested is handled with forceps and immersed one minute
in the acid; then it is removed and the acid washed off. The test is
applicable only to diamond, ruby, sapphire, spinel, emerald,
aquamarine, precious topaz, tourmaline, garnet, and kunz-ite, which
are unaffected by the hydrofluoric acid. The test is not applicable to
turquoise and opal, which are rapidly etched or eaten away by this
acid, nor to peridot and the quartz gems, as amethyst, false topaz,
crystal, agate, etc., which have their surfaces dimmed and require
repolishing.
