For metals it works quite differently. They also have an index of refraction, but for metals it's a complex number, which determines both the reflection and the absorption of light in the material. Octane doesn't support this directly, you have to approximate this by either setting an index of refraction greater than 1.0, or by setting it to 1.0 and using a mix texture + falloff map as glossy reflection.
Take for example gold. The index of refraction of gold is 0.17 + 3.14j at 650nm (see http://refractiveindex.info/?group=METALS&material=Gold). The complex part of this number is called the extinction coefficient. Octane doesn't support a complex IOR like this. Approximating with a IOR of 0.17 (without the complex part) gives a totally different effect, it would look more like an air bubble in water than like a metal.
The second issue, especially with gold, is that the IOR is dependent on the wavelength. There is no approximate formula for this variation like Cauchy's equation we are using in specular materials. You can look at the graphs on the linked page (the visible light range is between .40μm to .70μm). This gives gold it yellow or reddish color.
The third issue is that gold is usually alloyed with other metals, which affects the color (see http://en.wikipedia.org/wiki/Colored_gold).
The recommended way to render gold is to set the IOR to 1.0, diffuse color to 0.0, and use a mix texture + falloff map for the glossy color. The falloff map will simulate the Fresnel effect on the material.
And finally, if you render a metal material ball, you have to increase the glossy depth in the kernel settings or switch to path tracing, otherwise Octane won't render all the reflections on the metal.
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Roeland
