The process for making PMC Jewelry is different than casting with sterling (which contains copper).  The imperfections in each piece is because it is hand made, one at a time, rather than mass produced.  These originally designed silver jewelry creations are kiln fired to burn out binders and results in .999% pure silver.  The findings (ear loops and chains) are sterling.  Natural oxidation will o occur over time.  Use a silver polishing cloth to bring back the silvers natural luster.