The basic quality criteria for pearls:
- lustre,
- thickness and quality of the nacre layer (top layer),
- colour,
- clarity of the surface,
- shape,
- size.
The main factor determining the quality of a pearl is the thickness and quality of the nacre. The thickness of the layer determines the longevity of the pearl, while the quality of the nacre determines the light-reflectivity.
When buying pearls, it is also important to pay attention to the luster; it must be intensive. A pearl must glow from within. In high-quality pearls, there is a sharp contrast between the brightest shining part, which receives the light, and the shadowed part, which creates the illusion of a sphere inside the pearl.
Equally important is the colour of the pearls. Try a pearl on yourself (place it on your neck or face) and see if its colour matches your skin and hair. Inquire whether the colour is natural, especially for coloured pearls (grey, blue, black, etc.). An experienced gemologist may be able to help you determine the authenticity of the colour of your pearls (this is especially true for expensive black and golden pearls).
Do not forget to inspect your pearls for flaws. Although the characteristics of pearls are best seen against a light background, it is advisable to check for flaws against a dark background, under intensive and diffused lighting. Perfect pearls are very rare, and minor imperfections may be tolerated as long as the other characteristics (e.g. luster and the thickness of the nacre layer) are excellent.
The shape of the pearl is also important. There are three main forms of pearls: symmetric, spherical and baroque. Pearls of spherical shape are less common, which is why they are the most popular. Symmetrical pearls are both slightly oblong and pear-shaped pearls, which are somewhat cheaper than their spherical counterparts. Baroque pearls have an irregular shape and are therefore cheaper.
The size of a pearl is as important as its shape. Large cultured pearls are rare; hence, they are expensive. The price of a pearl starts skyrocketing with every millimetre its diameter exceeds 8 millimetres. In the shops, ask for the size of pearls and compare the differences in price between pearls of the same quality but different sizes. A double strand of smaller pearls may look equally beautiful and cost less than a single strand of larger pearls.
If you do not trust your own knowledge or if you are accustomed to rely on the opinion of experts in everything, you may wish to have an independent laboratory conduct a variation analysis.