I’m More Likely to Be Found in Your Shop than Your Cases

IF YOU REACH for me to use as a smelting flux when soldering gold, you are following in a metalsmithing tradition that dates back to the antiquities. Indeed, I take my name from the ancient Greek for gold and glue. That I have become something of a minor gemstone owes to my beautiful sky blue to greenish coloring, which often results in me being confused with malachite, turquoise or azurite. That I haven’t become a major gemstone owes to my softness.

Who am I?