Really, diamonds are only so expensive because of artificial scarcity; the De Beers cartel controls the supply — and has since the 1880s — and forces the prices up. Even were that not the case, dropping that box of a half-dozen, perfect, clear diamonds, each larger than the Star of Africa, onto the open market would depress prices hugely — and given that they're apparently so common they're used for snacks in the Harbinger's homeworks, setting up an import chain would destroy the market forever.
(Small diamonds have some real value — they're vital to some industrial processes, and tools made with inlaid diamond chunks are very useful for filing and such — and the cartel is bad enough by price-fixing those while supporting wars, slavery, and genocide. But gem diamonds are only worth anything at all because we've been told relentlessly that they're how you demonstrate love; the diamond engagement ring was pretty much invented by De Beers in the 1940s.)