When flush setting use a burnisher with a flat tip rather than a
rounded tip. The rounded tip will not allow you to burnish the metal
at the point of contact with the stone, which may cause difficulty
tightening the stone. A flat tipped burnisher makes full contact
with the metal allowing you to burnish the metal tightly against the
stone. A pointed tipped burnisher works well on diamonds, however the
pointed tip may scratch softer colored stones.
Brad Simon CMBJ