Answer: When he discovers the gold pieces in the shipwreck, he takes them, even though they have no worth on the island.
A person who is materialistic is one who values possessions and wealth to an extremely high degree. When Crusoe finds the gold pieces in the shipwreck, he takes them, even though he knows that these pieces have no worth in the island, and that there is nothing to buy in the island anyway. However, he values wealth so much, and wealth is attached to gold in his mind, that his instinct to desire gold trumps his logic.