Tarantino's co-writer, Roger Avary, says that when they wrote the script for Pulp Fiction, it was diamonds. Just plain old diamonds. Why didn't they just show that? Because Tarantino had just used a case of diamonds as a major plot device in Reservoir Dogs, and he and Avary agreed that it was "too boring and predictable" to do that again. So they decided not to show them, thus giving the whole thing an air of mystery.