Pronounced NAY-kur, this is more commonly known as "mother-of-pearl". This is the organic substance which is found on the inside of a mollusk (pearl, mussel, etc). The shellfish which lives inside the shell, secretes mother-of-pearl and coats the inside of the shell it lives in to create a smooth surface which they find more comfortable. The sharp edges of sand and other irritants which find their way inside can damage the soft tissue of the shellfish. This is how pearls are formed.
Man-made pearls, such as those made by brands like "Preciosa", are created using a crystal base over which several layers of nacre are applied, using a process which is very similar to the way a natural pearl is created. These man-made pearls are indistinguishable from natural pearls and are highly resistant to abrasion, alcohol-based perfumes and UV light. They can be created in many shapes.
Good quality man-made pearls, such as those created by Swarovski and Preciosa are formed using a good quality crystal as the base. The weight of this crystal gives the imitation pearl the realistic weight of a real pearl.