It also has extensive deposits of non-metallic minerals such as limestone , clay , marbles , guano , sand and gravel , and boulders .
Large deposits of copper , gold , zinc and chromite , manganese and nickel have been found in Isabela .
Isabela also has extensive deposits of non-metallic minerals such as limestone , clay , marbles , guano , sand and gravel , and boulders .
Large deposits of copper , gold , zinc and chromite , manganese and nickel have been found in Isabela .