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An increasing spiral motif, like a spiral shell. (The instructions are all in one image.)

While this is a pattern for a potholder, it could be used as a motif used for something else; the two-colour spiral is attractive in its own right.

While this is a pattern for a coaster, the spiral design could be used as a motif for something else also

How to crochet a spiral granny square using four colours. To make this in two colours, just simply follow the same pattern but use two strands in one colour, and two strands in your other colour. The order you place them will create different effects.

Written pattern can be found here: http://happyberrycrochet.blogspot.co.uk/2015/10/how-to-crochet-spiral-granny-square.html

The Crochet Charm: Spiral Flower Pattern. Pretty motif, but involves some sewing at the end.

A two-colour spiral is pretty, but tricky to start. This video tutorial is nice and clear.