66 A Book of Precious Stones
tions may occur in layers or irregularly; when in layers the layers are usually perpendicular to the faces of the prisms.
The
high esteem in which choice emeralds are held and the high cost of this
gem are due in great part to the rarity with which a gem approximating
perfection occurs. Most of the grass-green beryl crystals are cloudy
and dull; these disqualifications are due to fissures and cracks, but
also to infinitesimally small enclosures of foreign matter, either
fluid or solid, such as scales of mica. When clouded by fissures
emeralds are called by jewellers " mossy."
A
"perfect" (approximately of course) emerald-beryl stone is worth
nearly, sometimes fully, as much as a fine natural ruby and more than a
diamond—that is, a stone of one carat or thereabouts,—while large
stones are so rare that they bring fancy prices out of all proportion
to their size. The average emerald beryl fit for cutting is but a small
stone. Tradition and unscientific accounts tell of phenomenally large
emeralds, but one of the largest and finest actually known to exist
belongs to the Duke of Devonshire; this is a natural crystal,
measuring two inches across the basal plane, and
