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Compute the volume in cubic centimeters.
V = (18*2.54)*1.0*(0.1) = 4.57 cm^3
Then multiply that by the 19 g/cm^3 density for the mass in grams. It is about 8.69 g. That is about 1/4 Troy oz.
Most gold necklaces are not rectangular-prism blocks; they are chains, with void spaces to allow greater flexibility. The calculation indicated above is an approximation.