White marble is indeed a valuable resource! One can find it on most of the limited-time islands, including the Island of Bounteous Crops.

It can also be obtained on the following permanent islands:

— Home island (the territory behind the lock-stone).

— Island of the Eastern Bazaar (Level 16+). White marble located there is required to complete the island quest.

— Island of the Turtles (Level 21+).

— Island of the Oracles (Level 24+). White marble located on this island is also needed for completing its main quest.

— Island of the Olive Grove (Level 28+). After finishing Raphael's quest, you will also be able to find white marble on the missions’ islands.