of
Natural History, and in the United States National Museum, are
exhibited single crystals also of great size. That in Boston is three
and one-half feet long by three feet wide, and weighs several tons.
That in the National Museum weighs over six hundred pounds.
None
of these crystals is of a high degree of purity or transparency, but
the crystal planes, at least of the prisms, are well developed.
Beryl
crystals have no marked cleavage, except a slight one parallel with the
base. Where broken, the surface shows conchoidal fracture.
The
mineral is quite brittle. Some emeralds even have the annoying habit
of breaking of their own accord soon after removal from the mine. This
can be prevented by warming them gradually before exposing them to the
heat of the sun, or other sudden heat.
Beryl
and its varieties are dichroic ; i. e., the stones exhibit different
colors when viewed in different directions. This dichroism can
sometimes be observed by the naked eye, but better with the
dichroscope. With this instrument the twin colors seen are, for the
emerald, yellowish green and bluish-green; for the aquamarine,
straw-white and gray-blue; and for noble beryl, sea-green and azure.
The dichroism when seen furnishes a positive means of distinguishing a
true stone from any glass imitations.
The
varieties of beryl have not the brilliancy of the diamond, the double
refraction being weak and the dispersion small. They therefore depend
on their body colors and their luster for their beauty and
attractiveness. Fortunately they usually exhibit these qualities as
well by artificial light as by daylight.
Ordinary
beryl is a mineral of comparatively common occurrence, being often
found in granitic and metamorphic rocks, although that of common
occurrence is usually too "clouded and fractured to be of use for
gem-cutting. There are many localities, however, where beryls of gem
quality occur.
Of
the different varieties of beryl, the emerald is by far the most
important as a gem. Its pure green color, unalloyed by a single ray of
yellow, has ever made it an object to be sought for with avidity, and
it will probably always be the standard green gem.
The finest emeralds in the world come from Muso, a locality in the
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