TENACITY & TOUGHNESS

Tenacity or 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

Those 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.

THERMAL CONDUCTIVITY

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 cutting.

  Natural Gemstones