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A high strength with non-magnetic and non-sparking copper alloy with 0.5 to 3% beryllium and sometimes other alloying elements. It has excellent metalworking, forming and machining qualities.
You cannot silver solder beryllium copper - this is not true, you can, but the piece will lose the spring.
beryllium compounds are toxic and cumulative poisons, there are some safety concerns with handling its alloys. In solid form and as finished parts, beryllium copper presents no known health hazard. However, breathing its dust or vapors, as formed when machining or welding, will eventually cause serious lung damage. http://en.wikipedia.org/wiki/Beryllium_copper
Inhalation of such dust or fumes can cause serious pulmonary illness in a small percentage of people (around 4%). Since individuals at risk cannot be previously identified it is necessary to protect all potentially exposed persons. copperinfo.co.uk - Beryllium Copper H&S Notes [pdf]
See also: Copper.