Cloysters are human excrement in one of 2 forms, varying in quality and size depending on the type and maturity of the cloyster as well as its broad chemical compostion.
1. Inner cloyster - An internal conglomerate of fecal matter that may have taken years to accumulate in the lower bowel.
These are typically "high quality cloysters" and are usually much smaller in size and denser than outer cloysters.
2. Outer cloyster - often occuring on people who have not had a shower for a couple of weeks. These are also often referred to as "inferior cloysters" and are usually dry and light in colour.
Note: There may also be a book detailing the physical properties and variation of cloysters and details how to grow your own.
Inner cloysters are a relatively new discovery compared to outer cloysters which have been known for many millenia.