Crushed walnut and other such substrates are sold because people buy them without doing proper research, relying instead on pet shop employees to ensure they have the best stuff for their animals. Unfortunately not all of these employees have a clue what is good and safe for reptiles...and push selling store stuff as often they get paid more, the more they sell.
For most lizards...there is nothing sold at pet stores substrate wise that is safe. Calcium based sands are just as risky as the crushed walnut. Snakes are fine on shredded aspen or other hard woods.
It is always very important to research several sources before buying any animal, to find the best way to care for them and best supplies. Don't just rely on a single source. I researched several months before buying my first reptile. (a bearded dragon). This was especially good for me as the first reptile I wanted was a green iguana, till I researched them and found out they are alot more work than I expected, not to mention requiring a very large cage, can live 20 years, etc. A bearded dragon was a far better alternative.
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PHLdyPayne