They may be naturally occurring or synthetic. Nitrites are not naturally occurring in vegetables in significant quantities. These ligands can be naturally occurring or synthetic. The stilbenoids are naturally occurring stilbene derivatives. Historically, many of these sites were naturally occurring. Tannins are polyphenol compounds that are naturally occurring in apples. However, it is not naturally occurring. Of these, three are naturally occurring. Chitin is one of many naturally occurring polymers. Not all varieties of quartz are naturally occurring. It is also resistant to naturally occurring biodegradation processes. Most are naturally occurring geographical features, but some are artificial. It is the sweetest of all naturally occurring carbohydrates. "Pozzolana" are naturally occurring pozzolans of volcanic origin. Endocannabinoids are cannabinoids naturally occurring in the body.