Chapter 10 of Pearls A Practical Guide considers freshwater pearls.
Lusciously lustrous soufflé pearls. Unfortunately, such pearls have all
but disappeared. No farmers are growing them
Ablaze with colour – natural colour, not dyed, not in any way treated. smooth and shiny coin pearls
The best ripple pearls blaze with natural colour and incredible lustre. There are plenty of mediocre ripples – not a lot of lustre and wan colour.
Natural white pearls do exist but most white pearls are bleached in a mild peroxide solution (same stuff as for hair and disinfectant!) coupled with UV light. Here bottles of freshwater pearls sit in a dedicated rig in a Chinese family pearl factory.