It should move easily on the chain.
The clasp is secure yet easy to open and close.
The clasp is light enough that it doesn't pull to the front.
The setting focuses on the diamond or gemstone.
The pendant doesn't flip over.
It should sit perfectly in the notch at the base of her neck but also have an extra loop on the chain so the pendant can hang lower when desired.
Simple and classic is best for a solitaire pendant: platinum, 18k gold, diamond, sapphire, ruby or tsavorite. What to look for when selecting a diamond or gemstone will be another post.