How a pearl is specified?What are its standard specifications?

Question

How a pearl is specified?What are its standard specifications?

Answer 1

I know size is one.

smoothness too but I think there's another word for it.

Answer 2

Pearls are classified as an organic gemstone,(produced by

biological process,in this case animal)pearls consist of about 92%of calcium carbonate,or CaCO 3,in the form of aragonite crystals,held together by an organic substance,

conchiolin-about 6%-which is identical to the horny outer layer

of oyster shells,plus a small quantity of water-about 2%-,

Mother of Pearl has a similar chemical composition but with

less calcium carbonate,-about 66%-,and more water-about

31%,and is used as the nucleus of cultured pearls.

Pearls are undoubtedly the most costly and important of

"organic"gems.They have been known since time immemorial in the Orient andwere known to the Greeks and Romans, evidently following the conquests of Alexander the Great.

--Crystal System--

Aragonite crystallizes in the orthorhombic system

--Appearance--

Pearls are globullar,usually almost spherical cysts,which form inside the tissues of the mollusk,sometimes they are pear-,egg-,or bean shaped or display more pronounced irregularities consisting of roundish apophyses or even sharp

crests.The color is generally much the same as that of the inside of the oyster shell.Most pearls therefore,are whithe with a touch of gray to yellowish gray-white,but they may be

grayish,blackish,or iridescent from gray to green-blue-violet

and pink(the latter color  applies to rare pearls produced by

marine gastropode molluks of the Haliotis and Strombus genera)Pearls are composed of numerous,thin concentric layers,which are deposited successively by the mollusk("onion"-structure)To some extent the older the pearl ,the bigger it is,and the more numerous are the constituent layers

But in cultured pearls,which nowadays far outnumber the others,the inside consists of a spherical nucleus of mother-of-pearl,often taken from the shell of another mollusk,artificially shaped into a bead,but composed of flat,parallel layers,surrounded by a number of concentric layers of nacre deposited around it by the pearl-producing mollusk

--Physical Properties--

Pearls have a hardness of 2.5-4.5;but they are fairly resilient,

due to the organic substance they contain and their compact,

concentrically layered stucture.The density varies from 2.40

to 2.80g/cm3,but most of those used as gems have a density

of about 2.68-2.74g/cm3 in the case of natural pearls and 2.73-2.78g/cm3 in the case of cultured pearls.The refractive

index can only be measured by complex procedures

--Genesis--

Pearls form when a foreing body,such as grain of sand,or more often a small parasite,finds its way into a pearl oyster

which,in self defense, surrounds the intruder with a cyst and

goes depositing layer after layer of pearl over it,even when the intruder has been completely encapsulated and rendered incapable of doing any harm.Many mollusks produce pearls but the most important ones belongs to different species of

the genus -Pinctada(formerly known as Meleagrina),including

P. margaritifera , P. maxima , P. martensi, P. fucata, and P. vulgaris.These are medium-large bivalve mollusks(about 25 cm) of the Pteriidae family (Filibranchia order).Less valuable pearls are produced by many other mollusks,including some freshwater  bivalves and marine gastropods.

Answer 3

"Valuation factors include size, shape and quality of surface, orient, and luster."  Is this what you are after?

from Wikipedia at

http://en.wikipedia.org/wiki/Pearls