I can understand why someone would think this; after all, nowadays, a contralto or bass is relatively unheard of (tenors & sopranos seem to be reigning). The low voices are personally my favorite. Though I know a lot of lyrical/higher contraltos & basses. The coloratura & dramatic types are the ones that are actually rare. I also know people who think they are "basses" or "contraltos" but actually more of a dramatic baritone or mezzo-soprano.

But, a "true" bass or contralto will have all the qualities of that type in their natural voice; it will be deep, dark, and heavy. These types are very uncommon.