For example the top of the pendant in the link below:
Pendant itself is the word you are looking for,
Macmillan dictionary has the following definition of pendant.
Pendant > a piece of jewelry that hangs from a chain around your neck
Note: Gems and other precious stones are the ones which get set in the top of the pendant or rings. And they are just know by their names, like oh what a diamond ring, or a beautiful sapphire necklace.
Other related terms not to be confused with are :
Bail > a component of certain types of jewelry, mostly necklaces, that is used to attach a pendant or stone.,
Setting > what holds an individual gem,
Mounting > includes the collection of settings and connects to the shank of a ring or other jewelry
I believe it's called a bail.
Reference: Wikipedia entry for bail.
Bail is probably the correct technical term, but if you wanted to buy such an item (for crafting), the word you want is setting or mounting.
You can see some examples at this website.