Right, so this one isn’t too complicated: a parable about a pearl is called… The Pearl.
But there’s also some biblical stuff going on. It might be that Steinbeck is alluding to "the pearl of great price" from the Gospel of Matthew. (That the story was originally published under the title "The Pearl of the World" supports this argument.) In this biblical parable, a pearl-buyer sells all of his earthly possessions to purchase the world’s greatest pearl. The pearl, the Gospel explains, is like the Kingdom of God—men will abandon everything they have for God’s forgiveness.
There’s also a 14th Century epic (sort of) poem of unknown authorship called Pearl, in which a man dreams he goes to heaven, is distraught about losing his pearl, is told not to be distraught, and concludes that he should devote his life to doing God’s will... which is the greatest pearl of all and worth abandoning all one has.
If these references are intentional, the allusions are ironic one; Kino unwillingly loses what he has to "pay" for the pearl, a pearl which doesn’t represent the Kingdom of God and that brings him nothing but misery.