Factors which influence the rarity and value of pearls

Size
Whether natural or cultured, the larger the pearl the greater its value

Shape
A perfectly round pearl will command a higher price than another which is off-round or baroque in shape

Colour
There are a bewildering number of different colours available in "modern" cultured pearls, although natural pearls in antique jewelry are invariably white, grey to gunmetal, bronze, cream or yellow. Pearls with unsightly spots of brown will be correspondingly less valuable.

Purity
Since pearls are a natural orgnaic material, they are prone to irregularities, spots, blemishes and damage. Pearls will often wear down around the drill holes in necklaces whilst cultured pearls invariably exhibit blisters, holes and surface imperfections.

Lustre
The beautiful surface of a pearl is described as its nacre. The thicker the nacre, the higher the lustre and the greater the value.