Rug hooking is traditionally done with either wool strips or wool yarn. Other fabric types can be used, such as velvet, velour, wool roving, cotton, and synthetic fibers. It's very easy to mix both together in a project, and can offer additional texture and interest to a piece. Here are a few examples of mixing wool strips and yarn in one project.
A sparkly yarn added to a piece can add color, texture and a new visual element that blends in well with the existing piece. Another advantage is that yarn can be pushed and shoved into smaller places than wool strips. For this mat the sparkle outline was added between the grey border and the colored part of each square, after all of the hooking was finished. This can be easily done if the loops aren't packed too tightly. One thing to consider, particularly when working with synthetic material, is how the elements might age over time. This rug is now several years old, and the yarn has started to shed slightly. The yarn itself was a similar color to the wool strips (grey, pink and purple variegated). Don't underestimate how much yarn is needed for a project like this - I used more than one skein of this yarn to finish the project and had to hunt down a second skein.
Here's another example of yarn that was used to outline the purple diamonds. The color of the yarn and the wool strips were similar. Adding the yarn gave pops of turquoise and purple mixed with the regular strips.
Another way is to hook certain elements with yarn and others with wool strips. In this case the heart is hooked with yarn, and the background with wool strips. They are a similar size (three ply yarn and #5 cut wool strips), and were hooked at the same height, but these two elements could easily be hooked at different heights.
