Silver and Black Square Post Earrings

These silver earrings could be sterling, because they are tarnished and they polish up nicely, but there is no hallmark or 925 stamp on them, so I will just say they may also be costume jewelry. There is a black square in the center which may be a piece of onyx or obsidian inlaid in the center, but also it could be black enamel paint. Without a gem tester (mine broke 2 years ago) I cannot determine if it is a stone or not. These earrings measures 14.13 mm 5.55 mm thick, and weigh 7.09 grams.