Copied from the best site I have found on snake care: "Substrate Substrates are the stuff you use on the floor of your cages. For all my snakes I have found that Dri-dek (www.dri-dek.com} works great. Dri-dek is the rubber mats used in shower stalls at health clubs and such and I have even seen it recommended on some veterinary supply websites. It comes in many colors. They come in rolls or tile form. The rubber material is made of something they call oxy-B1 vinyl. This is a bacteria fighting material created to inhibits a microorganism's ability to absorb oxygen. This manufacturing process and will prevent mold, and bacteria. I love it. The tiles are raised off the ground by hundreds of pegs so the pee runs under and your pets are not always slithering through it and it hoses off easily for disinfecting. I layer newspaper underneath the mats. I have used mine for over a decade now. Not a single one has torn or broken. When it is possible, this is my substrate of choice. It may cost a bit up front, but you will save in the long run not having to keep buying replacement substrate. I avoid wood chips and products like “Repti-bark” from companies like Zoo-Med. This stuff kills countless reptiles every year yet they continue to make this garbage with a picture of an Iguana on the package. Let me paint a picture here for you. Your cage bottom is covered in beautiful “repti-bark” or some wood chip product. Your bearded dragon is ready for its dinner. You drop in a few crickets and he chases them down grabs a cricket along with a mouth full of this wood chip. It gets into his little tummy. It then lodges in his tiny intestines. It then slowly kills your pet. Same goes for your herbivores. Should a leaf or veggie fall out of his bowl he goes for it and get a mouthful. Some folks say they don’t feed their pets in the bark filled cage. This helps but many lizards just walk around all day tasting things and it sticks to their tongues. Got the picture? After soaking/freezing it for a month first to kill any mites I have used it for my Tarantulas but that’s about it. Cedar Shavings are toxic to many reptiles and animals. People like it because it covers oders so well. It is those fumes from the oils in cedar that cause irritation in the nasal linings of rodents and reptiles. By the way, if it smells bad clean it! Don’t mask the stink. I only use pine shavings on “Tribble” my chinchilla and “Mr. Whiskers” my hedgehog, never on reptiles. Although I do not have experience with it, the only wood chips I have heard are OK for reptiles are Aspen shavings and Cypress mulch. It does allow snakes to burrow if that is a natural behavior for your snake of choice but even these have their downside in that the tiny shavings can get in between belly scales and cause irritation. Of course, never feed your snake in shavings. If you are on a tight budget newspaper is still an affordable and safe substrate. Not too attractive but easy to replace when in a hurry and clean is important to your reptile. There are other products like butcher paper that come in rolls, some even have a waffle like texture for better absorbency. This is great too. Easy to replace, wipe down the floor with disinfectant and lay down new paper. I like this stuff. I used to use it but it does get expensive. If you only have a couple of snakes this may be the way to go for you. Some people like to use fake grass or “Astro-Turf”. This is OK. There are two health or safety issues I have with it. The edges can fray and get ingested by your pet. Lizard claws tend to shred it and they end up eating it. This may lead to potential health problems and I find it very difficult to keep clean. If you only have a couple of small snakes it may be worth the extra effort in cleaning to have a cage with what looks like a grass floor but when you get into the big guys it gets to be a bit much. Calci-Sand”, gravel and other sand like products are not as good as they may seem. Snakes get it lodged in their nostrils, heat pits and between belly scales. This leads to irritation, infection and even death. The packaging on “Calci-Sand”, states that reptiles can safely ingest it and it is supposed to break down in their systems but my vets and many posts on reptile message boards tell of dragons getting impacted from ingesting it. Just like wood chips it tends to clog up their digestive system especially on desert lizards that don’t require lots of liquid to drink. Now, I have heard that plain old silica free natural play sand and decomposed granite does not seem to cause as much damage. But if it’s not 100% safe I say skip it. http://www.snakebabe.com/gencare.shtml#Anchor--Substra-21663

Personally I avoid any form of shell or chipping, I use kitchen roll... they cant swallow it and become impacted, its cheaper, and easier to keep clean

We use cedar style chips for our snake(Fox),you know it,it's what is used in Gerbil cages..etc.it allows us to clean up after she does her business and still has enough 'roughness' for her to aid in shedding her skin,,last shed she needed rocks with jagged edges as her skin wouldn't come off clean,,it came off in small pieces(like hundreds of 'em),,I can't see Walnut shells as being a total negative but maybe others with more info can help you more.:)