Physical Properties of Gemstones




The Effects of Heat

When placed before the flame of the blowpipe, some minerals fuse entirely, whereas only the surface consistency of others is affected. The results are used in the identification of rough minerals. Such methods are useless in identifying fashioned gemstones, since their beauty and value would be partly or wholly destroyed. However, it is very important for the the repairman to be familiar with the effects of heat on various gemstones.

Of the important gem minerals, turquoise, emerald, garnet, quartz, peridot, zircon and tourmaline are likely to crack under heat unless well protested from direct blowpipe flame and cooled slowly. Turquoise is so vulnerable to direct heat that even the low temperature produced by a burning match may cause it to fly into may pieces. Fortunately, other gemstones are not as easily damaged by heat. In addition, a number of stones are likely to change color when heat is applied. These include amethyst, aquamarine, zircon, tourmaline and topaz. If subjected to sufficiently high temperatures (about 900°C) for a long enough period, diamond will first cloud and then blacken on the surface, and may even be totally destroyed by burning up.

From the number of stones affected, it can be seen that it is important for the repairman to be careful when sizing rings and doing other types of repair work involving the use of heat.

The Effects of Chemicals

Certain elements or combinations of elements, especially acids, have a visible effect on the surface of certain minerals, and more especially on the powder of a mineral. The effect of such chemicals is used to great advantage in identifying rough minerals, but would affect the beauty and value of a fashioned gem.

Hydrofluoric acid is sometimes recommended as a test. It dissolves glass but etches the surface of the quartz gems; therefore, it is of value number of gemstones are affected by acids, most transparent stones are affected very little. Many of the translucent and opaque stones are affected materially. Both natural and cultured pearls are dissolved by hydrochloric and some other acids. All acids affect pearl, coral, malachite, azurite and turquoise. The effect of hydrochloric acid is sometimes used to separate imitation pearls from cultured and natural pearls. A drop of the acid on any pearl will cause strong effervescence but will not affect the imitation. Since the acid will damage a pearl unless applied in a minute droplet and removed quickly, extreme care must be exercised in using the test.

Feel

Some minerals may be recognized by their feel, but the highly polished surfaces of gems so greatly lessens the possibility of using feel for identification that it is only mentioned in connection with a few stones: Jade (greasy), topaz (usually slippery), pearl (gritty to the teeth), some chalcedony (adheres to the tongue). Those who experiment over a number of years may develop a valuable sense of feel.

Odor

Some minerals have characteristic odors reminiscent of garlic, horse radish, sulphur, resin, etc, when heated, rubbed, breathed upon or struck. Rarely, however, does odor prove valuable in gem testing. Almost the only use is in distinguishing between amber and its substitutes.

Magnetism

A very few minerals in their natural state are capable of being attracted by a strong steel magnet; they are often said to be magnetic. These minerals are seldom of gemological importance. Hematite is sometimes slightly magnetic, but this is probably due to a small admixture of magnetite, which has the property of being attracted by a magnet. Magnetite, pyrrhotite and some varieties of native platinum are magnetic. The variety of magnetite known as LODESTONE exhibits the attracting power and polarity of a true magnet. These are sometimes called "natural magnets" and are occasionally used for pocket pieces.

Electricity

An electrical charge can be developed in many substances and in some degree, in all minerals by either friction or heat. Electricity developed by heat is Called PYROELECTRICITY. Tourmaline is pyroelectric, and topaz and amber are examples of materials that become high electrified by friction.

Fluorescence and Phosphorescence

Some minerals, when exposed to invisible ultraviolet or cathode rays, will emit visible light. This property is known as FLUORESCENCE.

It is named from the mineral fluorite, which often displays the phenomenon to a market degree. Almost every genuine or synthetic ruby fluoresces a bright red when exposed to ultraviolet light. When the glow continues, or "coats" after the exciting rays have been cut off, the phenomenon is known as PHOSPHORESCENCE.

Many diamonds fluoresce in ultraviolet light and some will phosphoresce. The significance of fluorescence in diamonds lies in the fact that most of the stones that do glow under ultraviolet light exhibit a blue color, The presence of ultraviolet in day light causes diamonds that fluoresce a strong blue to take on a blue cast that is not evident in artificial light (which lacks ultraviolet). Thus diamonds that fluoresce blue have a much better color in daylight than in artificial light. Some jewelers have used this property to advantage in creating interest in diamonds. Although it has little effect on value, it does make some customers feel that their diamond is unusual and unlike those of their friends.

Since various gemstones react differently under ultraviolet light, characteristic fluorescence can sometimes be used as a means of identification. Usually, however it is used more as an indication of identity rather than as a means of conclusive identification.