There are basically two types of pearl. Natural pearl (http://www.gemstonebuzz.com/natural-pearl) and cultured pearl (www.gemstonebuzz.com/cultured-pearl). 99% of the pearl in the market are cultured pearl and they comes in all colors (white, yellow, red, green, black, gray etc.). The process of manufacturing cultured pearl through oyster is same as natural pearl – the only difference is that, cultured pearl is formed due to the intervention of man while natural pearls are formed without intervention of man. However, cultured pearl will be as beneficial as natural pearl for astrological purpose.
Cultured pearls are cheap. You can get a good shape (almost round) at around Rs. 150 – 200 per cart. Also, it is not required to get it certified unless you’re buying from an unknown source as certification would be costlier than gemstone itself.