Hermione wrote:

"I had, it's true ... So there seem to be two main
sorts of stone balls, then: ones made in 20 minutes,
and natural ones that take thousands of years ...'

Yes, the Coasta Rican stone balls are madmade. Go read:

"Stone spheres of Costa Rica" - [en.wikipedia.org]

and "The Stone Balls of Costa Rica" - [web.ku.edu]

However, the stone balls found around Jalisco, Mexico
are natural in origin. Go see "Spherulite" at;
[en.wikipedia.org] and "Megaspherulites" in vol. 38,
no. 7, july issue of the BackBender's Gazette at
[www.hgms.org] , a 1.4 MB PDF file.

Yours,

Paul H.

"The past is never dead. It's not even past."
William Faulkner, Act 1, Scene III, Requiem for a Nun (1951)