Several yarn makers now offer yarn made of organic material.
While organic does not necessarily mean 100% environment-friendly - growing cotton uses a lot of water, be it organic or not, although organic farms strive to make the most of rain water and use less irrigation - it does entail that no pesticides were used, which is particularly significant for cotton, as 25% of the world use of pesticide is solely devoted to cotton!
A major brand that makes organic cotton yarn in France in Plassard. It offers organic cotton in sports weight, as well as recycled cotton.
(I doubt their recycled cotton is suitable for crochet though, as it is more loosely spun; you need an extra firm ply in crochet, as you handle the yarn in the opposite direction from knitters and therefore unwind it at every turn of the hook. This article explains it quite well. Plassard's "coton bio", OTOH, is quite crochet friendly.)
The pattern for this is "Eva's Shawl", one of the most popular crochet shawl patterns on Ravelry. It's free - here's the direct link to the PDF.
I added a shell border of my own - here is it (in the improbable case anyone's interested!):
Final row of shawl has to be row 3.
Row 1: ch 1, sc in beginning chain, sc in each DC, 2 sc in top of beginning chain
Row 2: ch 3, DC in first sc, ch1, skip next sc, ch1, DC in next sc across, except in the 3 ch-1 sp increases: DC, ch1, DC ch-sp. Put final DC in top of beginning chain.
Row 3: ch3 (count as 1 DC), 2 DC in first DC, sc in next DC, 5 DC in next DC across.
I crocheted this after I was done with my Mom's shawl, mainly during my commute from work (I work on the way to work, but am too knackered to work/read on the way back, so a repetitive crochet project is ideal).