The Black Pearl Mystery — Gertrude Chandler Warner

One warm night four children stood in front of a bakery. No one knew them. No one knew where they had come from.

Henry, Jessie, Violet, and Benny used to live alone in a boxcar. Now they have a home with their grandfather, and they are visiting Hawaii.

There they help pick pineapples on their cousin Mary's plantation, go snorkeling, and visit a volcano. But they can't figure out why no one will tell them the legend of the black pearl. And who took the diving maps that belonged to cousin Mary's husband? It looks as if the Aldens have another mystery on their hands!