Pearls are what non-boss Portrait Ghosts (excluding the pink and green Clockwork Soldiers and the purple vested twin, Henry) drop in Luigi's Mansion. A small pearl is worth 50,000G, half as much as a Gold Bar. A medium pearl is worth 100,000G, and a large pearl is worth 1,000,000G. A small pearl is awarded for every 10 HP sucked up within one turn. A medium pearl is awarded if Luigi sucked up 50 HP in one turn, then by sucking up every 10 HP after that. A big pearl is awarded if Luigi sucked up 90 HP within one turn. If Luigi collects all the pearls, he should have 4 small pearls, 4 medium pearls, and 1 big pearl, totaling 1,600,000G.
Pearls are what determines a ghost's portrait frame when captured. Getting only small pearls from a ghost results in a bronze frame; getting medium and small pearls results in a silver frame; getting a big pearl results in the best portrait frame, which is gold.
There are 76 small pearls, 76 medium pearls, and 19 big pearls in the entire mansion. However, Luigi can earn more small pearls at the expense of larger ones, up to 9 per portrait ghost (171 total). If Luigi collects all small and medium pearls after he collects all big pearls, he would earn a total of 30,400,000G for all pearls collected in the entire mansion.