To effectively utilize waste generated in the manufacture of artificial lightweight aggregate.

Melt deposits experiencing no granulation and adhering to the wall surface of a rotary kiln in the burning of expandable shale are crushed with a drill into crushed blocks of an appropriate shape. The crushed blocks can be utilized as a floating island having also a water purifying function, owing to small specific gravity, a void diameter and void distribution equivalent to those of a natural porous material, large water suction force by capillarity maintaining a wet condition, and a pH value equivalent to that of natural lava, all of which makes it possible for plants to grow well and for algae to propagate underwater, and owing to continuous pores offering a water purifying power of adsorbing particulates.