Other factors can be even more important depending on what you're making and how the finished item will be used. For a cosy TV blanket that will be used every day, you'll need a soft, warm, durable yarn that can be washed many times and still look good. If you can't find a yarn that you like at the exact gauge given in the pattern, then it's unlikely that it will matter if your blanket ends up a little bigger or smaller than the pattern. On the other hand, if you're making a fitted sweater, even one stitch out with your gauge could mean that it doesn't fit the way you're hoping. The techniques used in the pattern also affect the yarns you can use:
Modern Substitutes for Sixteenth Century Fabrics - Venus' Seamstress at The Realm of Venus
In this handy guide, learn how to substitute yarn fiber by exploring how different fibers behave. While this guide will help you learn how to substitute yarn fiber, please remember to always knit a swatch before you cast on your project. Want to know more about why you should knit swatches? Click here.
Silk & Nylon
Well, here's where I have to get a little blunt. If you want to look like a sixteenth century Venetian, you must find fabrics that will convey the right look and feel. Before you start jumping up and down and telling me you can't find, let alone afford silk, let me repeat myself: But if it's just an accurate period "look" you are after, then pattern, colour, and fabric weight will be more important than fibre content. There is no point in buying metres of pure linen with cabbage roses printed on it for a gown.