Dendritic chalcedony cabochon wrapped in pure copper. The brilliant blue patina comes from spending three days in a n ammonia/saline fume chamber. It’s hung on 20-24” adjustable cord. 
What we refer to as Dendritic Agates are actually Chalcedony, interspersed with dark black or brown inclusions of manganese- or iron-oxides that form in tree-like patterns (not bands). Like Agate, Chalcedony is also a microcrystalline form of quartz.