Texturing clay (any kind of clay) can be done cheap and easy using materials found around the house! This Instructible uses extremely old modeling clay to demonstrate.
Step 1: BoM
Misc. tools/materials around the house
Step 2: Floss
Create even tracks using floss. How deep the tracks are depends on how much pressure you use when putting the floss to the clay. Floss is also good for cutting clay (not canes).
Step 3: Staples
Both sides of a row of staples can be used to imprint on clay and still be usable for stapling later on :)
Step 4: Bottle Caps
Bottle caps, plastic or metal, make great designs and texture in clay. They can also be used over and over!
Step 5: Hardware
Hardware, such as screws and nails, also make some fantastic patterns. Just press the head of your nail or roll the body of the screw onto the clay and you're good to go.
Step 6: Buttons & Embellishments
Buttons, 3D stickers, stick-on jewels etc. make great texture tools!
Step 7: Paper Clips
Paper clips can be used as is or straightened out and twisted into your own designs!
Step 8: Writing Tools
Marker caps also make great textures. The first picture is two examples using both the body of a marker cap and the top of the cap to make designs. These might be my favorite. The top impression is great for steampunk designs.
The second two pictures are textures and designs all using different parts of a mechanical pencil.