Generally, a relatively simple chain works well with a diamond pendant, although it does depend on your taste! Some of the most popular choices are the simple cable chain, the slightly broader curb chain, or the thicker Spiga chain, which is made of teardrop or figure-of-8-shaped links.
The length of the chain is also important. A 16 or 17-inch chain will sit high on the neck, which works well for dresses with a high neckline. Alternatively, a longer chain, 22 or 24 inches, will hang below the clavicle which can suit a dress or blouse with a plunging neckline, to fully show the diamond off.