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20 G

This is quite a hard achievement, but if you really concentrate, you can get this in a surprisingly short time.
To get a black pearl, you need to create a flower of stars.
(You will also unlock Oyster-meister if you haven't already got it)
You need to do this 3 times. So that's 18 stars.

I'd suggest turning the patterns on in the settings, as this can make it a lot easier to see potential star flowers.

Star-walking is essential to move star pieces into place to get a black pearl.

What is Star-walking? If you have two stars stacked one on top of the other, you can grab the top star as the center, and turn it so that the other star piece moves. Swap sides and repeat until you've moved the stars into place.
Be careful of accidental combos below the stars you're walking!

You should try to make the starflowers as close to the center as you can, as this will make getting this achievement easier.