Can RFID work through metal?
There will be some attenuation from skin effect and Eddy Current losses dependent on c/fd ratio for d=thickness and speed of light c in air. So 100kHz will be attenuate slightly and result in loss of range, how much depends on material and thickness.

EDIT:  Skin Depth of Platinum is twice that of Cu. (better due to conductivity)

Skin depth is defined by 1/e or when the signal is 37% of the original.

\$\delta=\dfrac{1}{\sqrt{\pi f \mu  \sigma}}\$  for variables frequency, permeability and conductivity

.
 courtesy Wiki /Zurek's

Eddy Current Losses are defined by;

\$P =\dfrac{π^2B^2_pd^2f^2}{6kρD} \$
where
P is the power lost per unit mass [W/kg]
B is the peak magnetic field [T]
d is the thickness of the sheet or diameter of  the wire [m]
f is the frequency [Hz]
k is a constant equal to 1 for a thin sheet and 2 for a thin wire
ρ is the resistivity of the material [Ω-m]
D is the density of the material [kg/m³]