Two-Colour Diagonal Trebles Square
This pattern features in a number of different issues of The Art of Crochet with the differences being in the yarn combinations used.
Issue 14: Maize and Clover yarns
Issue 17: Vanilla and Turquoise yarns
Issue 23: Aubergine and Sailor Blue yarns
Issue 28: Lavender and Redcurrant yarns
It has been noted that this pattern comes up much bigger than the suggested 13 cm. You may find downsizing to a 3.5 or even 3.0 hook preferable.