Archive for February, 2009
Here we are referring to electrical properties in a general sense, ;md although these are of great interest, they are not important as distinguishing features of any particular stone. Of course, not all gems display this property, but a few develop electricity if friction, pressure, or heat is applied. When electrified, such stones will repel similarly and attract dissimilarly electrified bodies, while they will transfer their property to neutral bodies. Others exhibit polarity, that is, one end or side is charged with positive, mid the other with negative, electricity.
Friction will develop electricity in many minerals, the most prominent being topaz and amber among gem materials. The rubbing of amber, which results in negative electricity being produced, is well known to many who try to use this as a test for this material, although it is no proof of its identity. Incidentally, electricity is so named from the Greek word for amber, elektron, for this property was first observed by rubbing pieces of amber with silk by Thales in the Sixth Century B.C.
As a general rule, stones exhibit positive electricity only when polished, and negative electricity when unpolished, with the exception of diamond. Some crystals, when heated to a certain temperature, exhibit electrical properties which cause them to display polarity resembling that of a magnetic needle, and this property is termed pyro-electricity. It was first observed in tourmaline, but topaz, diamond, and many types of quartz also possess this quality.
If a crystal of tourmaline is heated to a temperature ranging between 10° C and 1500 C and then suspended by a fine silken thread, it will behave like a magnet having positive and negative poles. ‘As the crystal cools, its polarity becomes reversed. Tourmaline crystals, when perfect, are often terminated by different crystalline faces at the two ends, and other minerals which possess this peculiarity (known as hemimorphism) often display pyroelectricity. Topaz, prehnite, smithsonite, and axinite are examples.
Pyro-electricity in a stone may be detected by an apparatus in which a mixture of red lead and sulphur powders are blown through a fine sieve on to the stone. The resulting friction causes the particles to become electrified, red lead positive and sulphur negative, and these are attracted by charges of opposing signs. The color, red or yellow, of the powder will show the charge to be either positive or negative respectively.
Jewelers often use a small file for hardness testing. If the file cuts into the stone, a fine powder is produced, thus showing that the stone is softer than the file. The hardness of a jeweler’s file, which is finely cut and of hard steel, is about 6 1/2. It will mark feldspar but will not scratch quartz. Glass and pastes are marked by a file, their hardness varying from 4 1/2 to 5 1/2. For use in jewelry, stones should withstand this file test, but many well known varieties are softer, their other outstanding qualities overcoming this deficiency.
Tests on transparent and cut stones should be applied with discretion since a specimen may be easily disfigured by a deep file mark. It should therefore be carefully tested on an edge, preferably the setting edge. The sound made by filing and the color of the powder produced are also guides to hardness. But in general, this test is more useful with rough stones than with cut gems; there are other tests for distinguishing the latter without risking injury to the beauty and value of the specimen.
It must be added that some stones vary slightly in their degree of hardness. This will depend in a few instances on their place of occurrence, in others, on the direction in which they are tested, for instance, the hardness of diamonds varies slightly, the Borneo stones being harder than the South African stones. Different faces of the same stone may also vary in hardness, this feature being due to the internal molecular structure. An outstanding example is kyanite, a mineral whose hardness is 5 in one direction and 7 in another. But generally speaking, the limits of variation are very close to each other in most stones.