Jewellers' Network

Akoya Cultured Pearls

In Japan the pearl farmers begin to lift their harvest in December.  It is winter, the water is and nacre deposition has virtually stopped but they believe this is the time that the lustre of their pearls reaches its peak. In general the pearls of a “hamaage”, the Japanese name for a freshly lifted crop of cultured pearls, can exhibit a rather greenish in hue.

Once the flesh of the parent molluscs has been removed the whole crop is immersed in a saline solution. The pearls must be thoroughly cleaned before they undergo the preliminary sorting in which many substandard pearls are culled. The gem quality pearls destined to be sold undrilled are then removed which involves a further period of grading and selection.

Next those pearls that are suitable for making half-drilled pearls for jewellery applications are selected. In the case of these pearls the single drill hole may be placed in such a manner that it removes any minor imperfection. Then the usable pearls suitable for

full drilling are selected. The remaining 20% to 30% of the crop is discarded as being defective and therefore unusable.

The fully drilled pearls are bleached in a solution of hydrogen peroxide and distilled water before they undergo further grading and selection. A proportion of the creamy pearls with a good lustre and thick nacre may be chosen for treatment by irradiation or with silver nitrate to produce black pearls. Various processes, heat, light and dilute water-based tints are frequently used when attempting to establish uniformity of colour in those pearls that will make up necklace strands. The aim being to enhance as many of the pearls as possible. Ideally some will achieve the most desirable white rosé hue. Sieves are then brought into use to standardise the size of the pearls that are to be made up as necklace strands. Batches of the same diameter and general appearance are then given to the experienced girls working on sorting trays.

LEFT: Akoya pearL – rose

They take great care to ensure the matching of the pearls in each strand before the strands they have selected are shown to a supervisor for his final approval. The pearls are then threaded on a transport strands and made ready for shipping to markets around the world.   

The grading of pearls is a subjective and very complex art. The initial task that faces the gemmologist is to identify the nature of the pearl or pearls he is required to grade. He must determine if the pearls are marine or fresh-water, natural or cultured, nucleated or non-nucleated.

Of course he must first begin by ensuring that he is in fact dealing with pearls since there are many remarkable pearl stimulants to be found in the market today. Once he has established the species of mollusc that produced them and the basic nature of the pearls he must determine if they have been subjected to bleaching, dying, irradiation or any other form of artificial enhancement.

Suzhou Pearl Market, Shanghai.

Shape

Round or perfectly spherical these pearls have the most desirable shape. They are known in Japan as “eight-way-rollers” because they run true in any direction when placed on a perfectly horizontal sheet of glass.

Slightly off-round, off-round, these are pearls that tend to wobble either slightly or to a greater extent when put to this test.

Semi-baroque, pearls that are somewhat irregular in shape.

Baroque, irregular in shape.

Jumbo baroque, pearls in this group show a marked degree of irregularity to the extent that they may have tadpole-like tails etc.


Colour

The apparent colour of a pearl is made up of three components, body hue, mantle and orient. However a noticeable degree of orient is only found in those pearls that have a good coating and a fine lustre.

Pearls may exhibit a wide range of body hue, cream to yellow, greenish-white, white, pinkish, silver, bluish and grey. Bluish-grey pearls are often described as black.

Mantle in a pearl refers to a tone of another colour over laying the body hue of the pearl. It is known in the USA as ‘overtone’. If three pearls are brought together on a sheet of white paper and illuminated by a diffused overhead light the body colour can be seen in the gap between the pearls. The mantle/overtone will be the hue that can be seen immediately around the reflection of the light source on each pearl.

Orient is the green and/or purple iridescent sheen to be seen on very fine pearls, especially black pearls. It resembles the iridescent sheen to be seen on a pigeon’s neck.   

Lustre is a result of the fineness of the nacre platelets, their transparency and the total thickness of the nacre layers. The fineness of a pearl’s lustre can be judged by the sharpness of the reflection of a light source seen on the crown of the pearl. The more clearly defined this image the higher the lustre.


Nacre thickness

The “blinking test” is a basic test used to estimate nacre thickness. The strand should be held at eye level about 30cms from a light source and rotated. Blinking is the effect if the light either reflecting from the mother-of-pearl surface of the nucleus or being transmitted through its layers. The grader may also gain some indication of the thickness of the nacre by louping down the drill hole to see where the nacre ends and the dark conchiolin layer of the mother-of-pearl bead begins.

In describing a pearl necklace the gemmologist records the dimensions and number of the pearls, evaluates the matching and the presence or absence of blemishes and spots. Is the strand knotted and the details of the clasp and/or safety chain.

Descriptive terms for the length of strands. 

Choker:  +/- 36 cms
Princess: +/- 42cms
Matineé: +/-  54cms
Opera: +/-  72cms
Rope: +/-  96cms