A coated phosphor may comprise: phosphor particles comprised of a phosphor with composition (M)(A)2S4: Eu, wherein: M is at least one of Mg, Ca, Sr and Ba; and A is at least one of Ga, Al, In, Y; a dense impermeable (pinhole-free) coating of an oxide material encapsulating individual ones of the phosphor particles. The coated phosphor is configured to satisfy one or more of the conditions: (1) under excitation by blue light, the reduction in photoluminescent intensity at the peak emission wavelength after 1,000 hours of aging at about 85° C. and about 85% relative humidity is no greater than about 30%; (2) the change in chromaticity coordinates CIE(y), ΔCIE y, after 1,000 hours of aging at about 85° C. and about 85% relative humidity is less than about 5×10−3; (3) said coated phosphor does not turn black when suspended in a 1 mol/L silver nitrate solution for at least two hours at 85° C.; (4) said coated phosphor does not turn black when suspended in a 1 mol/L silver nitrate solution for at least one day at 20° C.; and (5) said coated phosphor does not turn black when suspended in a 1 mol/L silver nitrate solution for at least 5 days at 20° C.