The way the system works was kind of explained by Atani above, but to make it clearer:
- Importing an "object" as a clothing piece will try to conform said object to the character. Polycount does not matter, shape does not matter, it will try to make it conform.
- Importing as a clothing piece imports the object like a cloth. So, smooth and "soft" (just like cloth). Anything that should feel hard/rigid will completely loose it's original "aspect" using the clothing import system
For a necklace, you should make a custom unity asset, eventually with a physic simulation (like with hinges / joints) and use it as a CUA in VAM.