They said this provided the first direct evidence that plutonium-238 occurs in nature.

Inevitably, some of the numbers that occur in nature are prime.

Metallic lead does occur in nature, but it is rare.

Loose black lions of a type that didn't occur in nature.

It was the first color occurring in nature that I'd ever seen him dye his hair.

Francium-223 and francium-221 are the only isotopes that occur in nature, though the former is far more common.

The sugar was previously not thought to occur in nature at all.

But there is as yet no evidence from embryos that this occurs in nature.

Sometimes, particularly at higher levels, fences are designed that would not normally occur in nature.

None of the colors looked like anything that occurred in nature.