The Hebrew letters that you see on this impressive pendant are Shaddai, one of 72 Judaic names of God. It is common custom to inscribe one of these names in the middle of the Star of David. In Hebrew there is no star for the name of the hexagram – it is called Maged David, which means Shield of David. Now, David didn’t need any shield when he fought Goliath – in fact Saul gave him armor but he took it off. He believed that God, Shaddai, is his shield. Same logic here – the pendant doesn’t protect you from your enemies– God does!
Picture is courtesy of DrgnMastr who wrote under it that he got this pendant from his father when he was 16 just when he started his studies in the university.
The enamel was originally black but 35 years of wear has had it effect on it. The Hebrew lettering from right to left reads phonetically as "Sha-Da-Y", which I've been told mean "The One and only" in reference to G-d.