A pearl is a nacreous object produced within the soft mantle tissue of a pearl bearing mollusk. The mantle tissue is the part of the mollusk that is lined with many small, thin-walled blood vessels which extract oxygen from the water and expel carbon dioxide. All of those little blood vessels make it very sensitive, so if even a microscopic particle finds it way there the mollusk will react by releasing a calcium carbonate solution in the form of a mixture of aragonite and calcite that all sticks together with a substance called conchiolin. The combination of aragonite, calcite and conchiolin is called nacre. Nacre will coat the foreign body, and eventually, form a pearl. This calcium carbonate solution is the same material the mollusks' shell is made from. If you look inside a pearl bearing mollusk shell you will see a smooth layer of these tiny, colourful and refractive crystals coating the shell; commonly referred to as "Mother of Pearl". The colour of this lining will be indicative of the colour of pearls the mollusk will produce.

Not all mollusks produce pearls. The most popular varieties of pearls cultured today are the Freshwater Pearl; produced by the Cristaria plicata , Hyriopsis cumingi, and the Hyriopsis schlegeli mussels. The Akoya Pearl; produced by the Pinctada fucata oyster. The Tahitian Black South Sea Pearl; grown in the Pinctada margaritifera oyster. The White South Sea Pearl; produced by the silver-lip variety Pinctada maxima oyster. The Golden South Sea Pearl; produced by the gold-lip variety Pinctada maxima oyster.

A pearl possesses a unique combination of attributes that is unlike any other gem - it possesses luster and translucency which results in iridescence. Luster is produced from a combination of reflection and refraction of light. A smooth crystalline surface will reflect light, but layers of it will also take it in and refract the light. Most people don't realize that pearls' nacre layers are comprised of thousands of tiny crystals - when many layers of these tiny crystals are successively overlapped iridescence can occur that is referred to as 'orient'. You could compare the effect to soap bubbles where every angle shows a different hue that gives a rainbow-like effect. The thinner and more numerous the layers of nacre in the pearl, the finer the luster will be and the more likely it will be to possess orient.

When a layer of nacre is laid, it is comprised of thousands of tiny crystal platelets. When these platelets are laid evenly and smoothly without any raised or twisted interrupting the layer you end up with very smooth nacre. Many layers laid like this will create a smooth a lustrous complexion to the pearl.

Pearls have a hardness of 2½ to 4 on the Mohs scale, which means they are very soft and easily scratched. They usually have good toughness, but this can be variable due to aging, dehydration, and sometimes excessive bleaching during the initial processing. High heat can burn cultured pearls, or cause discoloration, splitting or cracking. Pearls are generally stable to light but heat from intense light can cause dehydration and nacre cracking. Pearls are attacked by many chemicals and all acids. Hair spray, perfume, cosmetics, and even perspiration can damage nacre. Pearls should never be cleaned in an ultrasonic or steam cleaner. Warm, soapy water is safe for an occasional cleaning, but if the pearls are strung, be sure the string is completely dry before wearing the pearls. For routine care, pearls should be wiped with a very soft clean cloth after each wearing.