TENACITY & TOUGHNESS
toughness is the ability of a stone to withstand pressure or impact.
Minerals which crumble into small pieces or a powder are said to be brittle.
If a gem bends but returns to its original position, it is said to be
elastic (mica, nephrite, jadeite); these minerals are tough and difficult to
break. The jade gemstones (jadeite, nephrite) are the toughest of all gems,
making them also difficult to cut. Talc and gypsum are examples of minerals
which are flexible. Ductile or malleable minerals are those (gold, silver,
etc.) which may be flattened out into thin sheets under pressure. The
brittleness factor of a gemstone is an important consideration in gem
cutting and polishing. Many gem crystals shatter or chip easily, and this
must be taken into consideration when cutting.
MAGNETISM & ELECTRICITY
stones which are attracted by a magnet are considered magnetic, such as
magnetite and hematite, which contain iron. Most minerals and gems are poor
conductors of electricity. Good natural conductors include native metals and
minerals with a metallic luster (pyrite). Natural blue diamond is a
semi-conductor. Some stones, such as tourmaline, become electrically charged
when heated and are said to be pyroelectric. Tourmaline is also
piezoelectric; it becomes charged if stressed at certain points along the
crystal. Quartz is an important piezoelectric mineral and this factor is
what makes it useful in electronic circuits and photoelectric processes.
Amber is triboelectric; it develops a negative electric charge when it is
rubbed and attracts small fragments to its surface.
Some stones are good conductors
of heat, such as quartz, which draws heat away from the body when held and
thus feels cold to the touch. A poor thermal conductor, such as amber, feels
warm to the touch because it does not conduct heat away from the body. The
surface of a genuine gemstone will de-mist more rapidly than that of glass
or an artificial stone. Thermal conductivity should also be considered when
cutting gemstones, as some stones will need a cooling-off period during the