I've been making those for years Woonilsra, I've just never knew what they were called. I have only made them from sand though.
I use essentially the same method as described on the link, except I deviate after step 3 by dumping a small pile of dry sand over the sand ball to draw the moisture out. After as few as 3-5 minutes of being buried, and after clearing the dry sand that clings to the ball off, (brush it off until you reach the more moist inner ball.) you have a slightly moist sand ball leather-like in stiffness at this point. I usually stop here.
Since I usually make them from sand (when I'm at the beach), I've only made the rough shape. (I never considered polishing something as fragile as a sand ball.

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Now I wanna try making a polished one. Thanks a lot.