Yarn

Yarn for crochet is usually sold as balls or skeins (hanks), although it may also be wound on spools or cones. The thickness or weight of the yarn is a significant factor in determining the gauge, i.e., how many stitches and rows are required to cover a given area for a given stitch pattern. Thicker yarns generally require large-diameter crochet hooks, whereas thinner yarns may be crocheted with thick or thin hooks.

There have a lot of yarn types and one of them is plant based fiber (Egyptian cotton, Pima cotton, American cotton, Linen, Bamboo bast). Cotton yarn has minimal elasticity unless blended with other fibers. Pure cotton is useful for projects that require structure such as purses and tote bags, placemats, and other utilitarian items. So, for my project I prefer cotton yarn because it soft and also suitable use directly on skin. It is expensive than synthetic yarn (acrylic, nylon, rayon) but cheaper than animal based fiber (Merino wool, Icelandic wool, Mohair, Cashmere, Alpaca, Angora, Silk). Depends on material and your budget, you can buy any of those yarn for your project. 

Source: Wikipedia

These are my cotton yarn collections;