A freshwater pearl is a pearl that is cultured in a mussel in a lake, whereas the traditional cultured pearl is cultured in an oyster in the ocean (salt water pearls are called Akoya pearls). Freshwater cultured pearls are less expensive than the Akoya pearls.
Freshwater pearls come in many shapes from round to button shapes, from flat flakes to rice shapes and in many colors. As with the salt water akoya pearl the round shapes are generally more expensive. So they are classified by origin, then graded by size, shape, nacre thickness, color, luster, surface clarity and matching. These qualities are not considered equal. Some factors will be weighted to give them more influence in arriving at a final grade.
Pearl luster is the quality and quantity of light a pearl reflects from its surface or near surface. It is a complex effect, quite different from the luster of surface of polished gemstones. It is produced partly by the surface reflection of light and partly by a subtle iridescence. This iridescence is caused by diffraction at the edges of overlapping platy crystals of aragonite and interference when light penetrates some of the thin plates,then reflected back to the surface.
Interference will take place on the surface of a freshwater pearl due to refraction and reflection of light on the thin alternating layers of aragonite and organic substance. This is a complicated optical phenomenon which occurs on a curved surface and may continue into deeper layers. This explains why individual pearls show different degrees of iridescence.
The color of pearls changes with the light source under which they are viewed; white light is preferred for color grading. When us daylight, the best results will be obtained under the light of a slightly overcast northern sky of medium latitude at noon. The direct sunlight should be avoided. The fine color of a pearl depends on the type and thickness of the prismatic and nacreous layers, in which conchiolin is the organic matrix. Colored pearls jewelry either have distinctly dark colors like brown, reddish-brown, grey and black or tints of these colors, which may appear as yellow, golden, bronze, green or orange hues.
Freshwater pearls’ sizes range from 2mm to 15mm. The average size is still from 4mm – 5mm to 6mm – 7mm. Sizes of 8 – 9mm have increased over the last years but account for only about 0.01 % of production for really round shapes. Sizes of over 12mm are rare.
