[VadeRetro]

Does lime occur naturally, too?

Yes, whether you mean unslaked lime (CaO) or slaked lime (CaCO₃). CaO is less stable (strongly alkaline) and will form CaO₃ with CO₂ in the atmosphere.

[The Electrician]

From the context of the article, I would guess that they were referring to CaO. But if it does occur naturally, then I withdraw my objection. Anyway, the article mentions that they ditched the lime hypothesis and moved on to borax as a more suitable material. BTW, thanks for the info.