  We present a comparison of the gas and dust properties of the dense
interstellar matter in six nearby star-forming regions (d<500 pc): rho Oph, Cha
I, R CrA, IC 348, NGC 1333, and Orion. We measure from Chandra and XMM-Newton
observations the X-ray absorption toward pre-main sequence stars (PMS) without
accretion disks (i.e., Class III sources) to obtain the total hydrogen column
density N_{H,X}. For these sources we take from the literature the
corresponding dust extinction in the near-infrared, A_J, or when unavailable we
derive it from SED fitting using the available DENIS, 2MASS, ISOCAM and other
data. We then compare N_{H,X} and A_J for each object, up to unprecedently high
extinction. For the rho Oph dark cloud with a relatively large sample of 20
bona-fide Class III sources, we probe the extinction up to A_J <~ 14 (A_V <~
45), and find a best-fit linear relation N_{H,X}/A_J = 5.6 (+/- 0.4)x10^{21}
cm^{-2} mag^{-1}, adopting standard ISM abundances. The other regions reveal a
large dispersion in the N_{H,X}/A_J ratio for each source but for lack of
adequate IR data these studies remain limited to moderate extinctions (A_J <~
1.5 or A_V <~5). For rho Oph, the N_{H,X}/A_J ratio is significantly lower (>~2
sigma) than the galactic value, derived using the standard extinction curve
(R_V = 3.1). This result is consistent with the recent downwards revision of
the metallicity of the Sun and stars in the solar vicinity. We find that the
rho Oph dense cloud has the same metallicity than the local ISM when assuming
that the galactic gas-to-dust ratio remains unchanged. The difference between
galactic and local values of the gas-to-dust ratio can thus be attributed
entirely to a difference in metallicity.
