This is a direct copy from the website rules:
"You may use any kind of materials and/or textures supported by your 3D program or renderer. Special effects such as fur, fire, water, gas, translucency, dispersion, radiance, displacement, etc, are not only permitted, but also encouraged. The grid plane must remain mapped with the provided grid texture and it may not be edited in any way (texture included). Please do not modify the grid plane, as it is required for consistency."