Lab Identification Methods

Unaided Eye

Observation should be the first step to start checking pearls and be the basis of gaining details to state an opinion. Roll over the pearl under the white light, careful examination of external and internal features of the pearl can provide significant information leading to the identification of the pearl and the detection of treatment.

With unaided eye, imitations of pearl are usually easy to recognize once the pearl tester is familiar with their surface textures. Cultured pearls with a thin pearly layer can be recognized without magnification, as the nucleus is usually visible to the naked eye. Mabe pearls are recognizable to the naked eye by the mother-of-pearl plate or bead on their lower side.

It is easy to see from the underside of a cut pearl if it is natural or cultured. Natural half-pearls show a concentric ring structure. Cultured pearls show either a mother-of-pearl flat or striped appearance, depending on the direction of the cut.

The ‘tooth test’ is another easy way to distinguish natural or cultured pearls from imitation pearls. Both natural and cultured pearls feel ‘gritty’ when rubbed across the teeth; imitation pearls feel smooth.

10X LENS

Experience pearl tester usually takes the test step start from 10X lens or jeweler loupe. This is the most common way to check a gem’s details without any damage.

For instance ‘10x’ (‘ten times’ or ‘ten power’) means that when examining an object with such a lens, the resulting angle of view is 10 times greater on your retina than that of the unaided image. The 10x lens is easily transportable, inexpensive and helpful for observation of pearl’s features.

It is important to use a good quality, well-corrected lens. A triplet arrangement eliminates color fringes and prevents distortion.

Under a 10x lens, the structure of overlapping platy crystals of calcium carbonate gives rise to a distinctive stepped surface. This serves to separate natural and cultured pearls from bead imitation. The surface of imitation pearls has a smooth to finely granular appearance, which contrasts markedly with that of natural and cultured pearls. Normally, gas bubbles are visible in glass imitation pearls.

If internal structure can be seen, it may be used to separate natural pearls from nucleated cultured pearls, which have a relatively large bead nucleus coated usually with less than 2mm of nacre. Half pearls may be inspected directly; the bead in a cultured pearl is usually white in color, and may display banding. Natural pearls show a concentric structure and the inner layers may be of a ‘warm’ color, such as yellowish, brownish or black.

In the case of drilled pearls, it may be possible to detect the same features by careful inspection of the drill hole, which must be clean. It may be expected that nucleated cultured pearls will show a sharp demarcation between the white shell bead and the outer nacreous layer. This junction may be enhanced by the presence of a pink dye, which is often used on cultured pearls before they are marked.

Natural pearls may show a succession of boundaries between the concentric layers, and it is unusual for these to be as obvious as the junction seen in many cultured pearls. The interior of the pearl may be yellowish or brownish. But thick skinned cultured pearls may show two or more concentric boundaries and, conversely, natural pearls may show only one visible boundary.

So the tests provides that it is stated the results of these observations may constitute an opinion, rather than proof.

The optical Microscope

The microscope enables a gemologist to quickly and correctly determine if he is dealing with a pearl or another product. However, microscopes are not always sufficient to distinguish between a natural pearl and a cultured pearl.

Both natural and cultured pearls show a characteristic surface structure of wave-like relief lines, which correspond with the border lines between the aragonite platelets on the pearls’ surface. A 200x magnification reveals the distinct relief pattern that is reminiscent of a topographical map.

The exact pattern of a pearl’s surface can only be recognized under an electron microscope, which reveals the densely connected hexagons of the aragonite platelets. The borders and differences in height between the individual platelets lead to the relief lines just described.

Like 10x lens, this method must be applied with care. Natural pearls may show growth zones and deposit of conchiolin, which an inexperienced observer may mistake for the borderline between nucleus and pearly layer.

Mabe pearls often have gas bubbles underneath their thin upper nacreous layer if they are examined under the microscope.

Endoscope

Only fully drilled pearls can be tested with this machine. It is very effective in distinguishing between nucleated cultured pearls and natural marine pearls.

The endoscope enables to view the behavior of a ray of light which is introduced into the drill hole using a hollow, ultra-thin needle (the diameter is about 0.3mm). The ray of light enters the needle from the light source and hits the first mirror, from where it is reflected on to the wall of the drill hole.

But now it is rarely used and is replaced by diaphanoscope which based on the endoscope principle. Diaphanoscope uses a laser beam which is largely reflected from the surface of natural pearls, and it is also suitable for undrilled pearls.

X-Radiography

This is one of the commonly used methods of identification today. The pearls are placed on a piece of photographic film and exposed to a beam of x-rays.

• Nucleated cultured pearls will show a pale, perfectly round mother-of-pearl bead, surrounded by a darker ring of conchiolin and nacre;
• Natural pearls may show one or more rings of conchiolin, usually of a less regular outline than those in cultured pearls;
• Non-nucleated cultured pearls normally show an irregular cavity at the centre;
• A glass-centre imitation pearl will show the white sphere and its dimpled surface.

X-Ray Diffraction

Lauegrams are patterns produced by a narrow beam of X-rays directed through the centre of a pearl. Natural pearls will show a six-fold pattern in any direction. Cultured pearls produce a six-fold pattern in one direction only, at right angles to the layers of the nucleus. They produce a four-fold pattern parallel to the layers.

X-ray fluorescence

Most of the natural saltwater pearls don’t fluoresce under x-rays. Freshwater pearls and freshwater cultured pearls form Japan, China and America generally show a strong fluorescence which is accompanied by distinct phosphorescence.