Spectral reflectance of diverse material

Enrique,
By "Fresenel reflection" you mean the specular relection from a smooth surface, as opposed to diffuse reflection?
Depending on the wavelength you are interested in, you might also try books on radiation heat transfer, such as Siegel and Howell. Also, there is a lot of information in the Infrared Handbook, (Wolfe and Zissis, eds.) and its successor, The Infrared & Electro-Optical Systems Handbook, a 7 volume set of books edited by Zissis.
Olin Perry Norton

That is right, I mean first-surface reflections.
I am interested in the visible light range, i.e. 300-700nm.
Could I find the dielectrics I am looking for in them? rutile, diamond, calcite, glass
Thanks anyway.
Enrique

[snip]
Dear Enrique,
You probably know this, but just in case, remember that there are formula that let you calculate the reflection coefficient if you know the index of refraction.
So, you might try looking for refractive index data in addition to data on reflectance. Particularly for a common optical material like glass, refractive index data should be readily available.
(Incidentally, the word "glass" covers a lot of territory.)
If you pick a particular type of optical glass, the manufacturer should have plenty of data available on the refractive index. Look at example.
Schott BK7 is a common optical glass. If you Google "refractive index Schott BK7" (without the quotes) you will find a lot of information.
I tried Googling "rutile refractive index" (without quotes) and found this: "Refractive indices of rutile as a function of temperature and wavelength," J. Rams, A. Tejeda, and J. M. Cabrera, Journal of Applied Physics (1997) Volume 82, Issue 3, pp. 994-997.
"The Infrared & Electro-Optical Systems Handbook," Volume 4, "Electro-Optical Components," edited by William D. Rogatto, in Table 1.52 on page 46 gives the refractive index of diamond from 480 nm to 656 nm.
Calcite will be a challenge. It is birefringent, so you may have to rederive the formula for the calculation of the reflection coefficient from the refractive index.
Olin Perry Norton

Thanks a lot for the answer, it is so helpful!
Yes, but I was not so sure about myself, so I would have preferred to get data from proper sources.
However, I followed your advice, and got the refractive indices of glass and rutile from the sources you mentioned. I computed the spectral reflectance myself, and they look correct! I am very pleased, thanks a lot.
"The Infrared & Electro-Optical Systems Handbook," Let's forget about calcite then. I have not looked at the book for diamonds, but I will. I could also replace them with other dielectrics. Do you know any whose spectral reflectances/refractive indices are readily available?
Thanks again
Enrique

[ Dear Enrique,
People who design optics must have access to compendia of refractive index data that include all commonly used optical materials. I'm not one of these people, so I can't really tell you the best place to look.
The Melles Griot web site I referred you to earlier and the web sites of optical glass manufacturers (like Schott) have tables of refractive index data for the materials they sell.
I'm not sure what your intended application is. If you want a selection of different materials with different reflectivities, you might be able to use different types of optical glasses.
They are readily available, not too expensive, and you can buy them already polished and optically flat. Using an amorphous material like glass would also eliminate the problem of anisotropy and having to determine how you're oriented relative to the crystal axes.
There seems to be an enormous literature on the refractive indices of various materials. I searched for refractive index on the NIST web site and came up with:
Shannon, Robert D., et al., "Refractive index and dispersion of fluorides and oxides," Journal of Physical and Chemical Reference Data, Vol. 31, 2002, pp. 931-970.
Google is tremendously helpful for finding this type of information. I randomly picked sodium chloride as a common chemical compound and Googled "refractive index sodium chloride" (without the quotes) and immediately found a table of refractive index values.
"The Infrared & Electro-Optical Systems Handbook," Volume 4, "Electro-Optical Components," edited by William D. Rogatto has tables of refractive indices of many different materials, especially those used as infrared windows and lenses.
Olin Perry Norton