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15 G

To get this achievement you must surround one hexagon piece of any color with 6 other pieces that are all the same color (see achievement pic for an example).

What this does is create a Starflower piece that allows you to rotate the 6 pieces surrounding it without needing to have made a combo for the move to stay.

Anyway, you'll probably get this as you progress through Marathon mode. Just keep your eyes peeled for a group of same-colored pieces.