2 Major mistakes...
Pearls are kind of mysterious because of the colour and lustre it has. We describe the lustre as 'orient' effect or 'iridescence'.
One major mistake when buying pearls is to see the size of the pearls. Larger doesn't mean better. In fact, some pearls though large, have only a very thin layer of nacre. It is large because of the bead nucleus used to produce the pearl.
The other major mistake for consumers is the failure to ask for the origin (or the make) of the pearl, whether it is south sea pearls or freshwater pearls. Instead, they ask about the price first. Important factors to consider first should be authenticity and quality of the gem material.
To better understand and appreciate pearls, there are 7 factors to consider when looking at pearl quality: Shape, Lustre, Surface Quality, Nacre Thickness, Size, Colour and Matching.
The most commonly appreciated shape is round. If the pearl is round, we have to consider the percentage roundness of the pearl. Generally, south-sea pearls are almost perfect in roundness, as these are cultured with round mother-of-pearl shell beads. Other shapes include button, tear-drop, baroque, oval etc.
Picture source: http://www.pearlotica.com/pages/Pearl-Shapes.html
Lustre in gemology is defined as the quality of light reflection on the surface of a material. It can be dependent on the quality of polish and the hardness of that material. Lustre for pearls is described as 'orient'. Sometimes, it is simply described as 'pearly'. Pearls have a hardness of between 2.5 to 4.5 on Mohs' Hardness Scale. It has a range of hardness because of the quality of nacre. Pearls with higher lustre has higher durability compared to pearls with low lustre as it can get damaged by scratches or other physical forces more easily.
Picture Source: http://www.gia.edu/pearl-quality-factor
#3 Surface quality
In other words, blemishes on the surface of the pearl. Scratches are easily seen on pearls with poor lustre, or pearls with thin nacre layer. Depending on the condition of growth, pearls with poor surface quality can also appear to have lots of pitting or 'pot-hole'-like blemishes. The more numerous there are of these blemishes, the lower the surface quality of the pearl. This factor is also the most obvious and irritating to the consumer's eyes. There is no pearl that is 100% perfectly free of surface blemishes. Choose one that is pleasing to your eyes.
Picture Source: http://bit.ly/1xIoCRs
#4 Nacre thickness
Pearl nacre is essentially the layers of mineralised (or crystallised) calcium carbonate and conchoilin formed during the formation of the pearl. Pearls are formed due to an injury in the oyster or mussel. As these are live animals (the oysters and mussels), they have a natural protective mechanism that will protect them from external harm. If a grain of sand gets into the shell, the animal will natural reject the 'intruder'. Pearls are formed when these 'intruders' cause an injury to the animal tissue.
Nacre thickness and pearl lustre is very closely related. Poor lustre is often due to thin nacre. Therefore, make your observation by looking at the lustre, but not forgetting that there will be times when thin nacre produces good lustre due to high quality of production.
Another factor that is most appealing to consumers is the size of pearl. The larger the pearl, the higher the value due to its rarity. Even for cultured pearls (so called with 'human intervention'), it is rather difficult to produce a large pearl more than 15mm.
Pearls sizes are usually described in millimeters, by measuring the diameter of the pearls. In the trade, pearl weight is sometimes used and the unit of measurement is in momme, an ancient Japanese measurement still used in today's context.
1 momme = 18.75 carats = 3.75grams
Picture Source: http://theonlinejeweller.org/saltwater-akoya-pearls/
Tip: If you'd like, bring along a measuring guage to check the size of the pearls!
Colour of pearls can range from white to gold, green to black. Due to the nature of pearls, it most often has secondary colours. Black pearls are rarely pure black for example. They always come with a secondary colour of green, blue or purple. Time is required to train the eyes for the colour grading of pearls. Our eyes become 'white-balanced' after a while when looking at something for a period of time. Hence, to determine the colour of a pearl is not an easy task. The picture below is an example of colour variation in pearls.
Picture Source: Far East Gem Institute gemstone identification course ppt
Tip: Colour preference is a personal factor. Make a choice base on what you like, rather than asking what is the best colour.
#7 Matching (for pearl strands and earrings)
In pearl strands, earrings or any other multi-pearl jewellery, the value increases as the consistency of the pearls increases. The more matching the pearls are, the more valuable it is. This is due to a few factors like the rarity of getting a strand of 40 pieces of pearls that have the same size, same colour, same lustre, same nacre quality and same surface quality.
It is difficult to produce (culture) a whole lot of pearls with the same quality factors. The effort to collect and put them together is immense and requires a trained and experienced eye to do so, especially in matching the colours. The longer the strand and the larger the pearl, the harder it is to put together.
Picture Source: http://nyti.ms/1IWOqiV
Finally, in order to give a pearl its value, the authenticity is very important, other than the 7 factors mentioned. When we talk about authenticity, what we really mean is the originality of the pearl. If it is a natural pearl, is it a natural-natural pearl? Or a freshwater pearl? Or a saltwater pearl (south-sea pearls as some put it)?
Another consideration is treatment. Has the pearl been treated in any way? Some treatments are: dyeing, coating, irradiation, heating etc. For a more informed purchase, ask the jeweller or pearl dealer for these information.
Check out the recent interview by May Seah, Senior lifestyle journalist of Channelnewsasia HERE. The Gem Museum founders Ms. Loke Huiying and Mr. Tay Kunming shared their gemology knowledge with readers on "How to Buy your First Real Pearls"?