Before diamonds can begin growing deep underground in Earth’s mantle, they need a little zap from an electric field, a new study finds.

In lab-based experiments, scientists mimicked conditions in the mantle — the layer just beneath Earth’s crust — and found that diamonds grew only when exposed to an electric field, even a weak one of about 1 volt, according to the study, which was published online Jan. 20 in the journal Science Advances.