mach diamond
 n.— «Nerves jangling, he awaits the moment when Carpe Diem, his homemade 18-foot-long rocket, hurls itself heavenward with 737 pounds of thrust, shockwaves—or “mach diamonds”—surging from its supersonic exhaust.» —“A Cult of Backyard Rocketeers Keeps the Solid Fuel Burning” by Patricia Leigh Brown in Gerlach, Nevada New York Times Oct. 14, 2006. (source: Double-Tongued Dictionary)

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