Gold

K = Karat
KP = Karat Plumb (exact amount of gold)
K.P. = Karat Plate
GF = Gold Filled (heavy plating), usually has a fraction, i.e., 1/20 10K GF
GS = Gold Shell
HGE = Heavy Gold Electroplate

Decimal equivalents of karat ratings:
.375 = 9K (England and Canada)
.417 = 10K
.500 = 12K
.583 (.585) = 14K
.625 = 15K
.667 = 16K
.750 = 18K
.833 = 20K (Asia)
.917 = 22K (Asia)
.999 (1000) = 24K pure gold

Silver

925 (.925) = Sterling
925 NV = Nevada Silver, made in England(?)
835 (.835) = a lower quality of British Sterling Silver
Sterling
Taxco
MEXICO
STER
.999 Fine = Pure Silver

Platinum

Pt (atomic symbol)
Irid. Plat.
Pt 900
Pt 950
PLAT

Other

316L = High-grade Stainless Steel