Rocks as mulch around plants work well from a decorative standpoint and with the proper material underneath, keeps weeds down. Because rocks are an inorganic mulch, they don't contribute to the soil, but on the plus side they will not decompose. The choice of rocks includes, marble chips, river rock, pea gravel and even lava rock will work. The material underneath the rocks keeps the rocks from mixing with the soil. Once the rocks mix in, it becomes difficult to separate them and you will need to replace the rocks. Eventually, a set-up with no material underneath results in a deep layer of rock that supports weeds.
Landscape fabric allows water and fertilizer to get to the soil but will keep weeds down. It comes in a roll and is lightweight and easy to handle. The fabric's ability to suppress weeds depends on how tightly the mesh is woven. Begin with weed-free ground for best results.
Plain black plastic helps the soil retain moisture. The dense plastic keeps weeds down effectively but stops air movement as well as water. This causes root rot if you have plantings in the area covered by the plastic. It does not decompose quickly, but if you need to remove the plastic it is bulky and cumbersome to dispose of. If you decide to use plastic around plantings, poke small holes with a screwdriver to allow rainwater in.
Cardboard or Newspaper
Cardboard or newspaper can effectively suppress weed growth. This material also is readily available for recycling in the garden. Cardboard and newspapers degrade over a season or two and lose their weed-suppression ability. While these materials work for the short-term, they do not work well for a permanent area of rocks.
At the time of publication, the cost for a roll of 4 foot by 220 foot of polypropylene landscape fabric is $29.97. This material comes in a variety of roll sizes, so consider size when pricing. The plain black plastic often comes with a smaller price tag, but the fabric lets air and water through. You can obtain cardboard and newpaper almost free, but it will cause you work later, which may be enough to negate the cost advantage.