If my verbal description was gobbledygook, here is a Poser render demonstrating the difference with some highly accurate shaders I built. (Click for full size)

One candlestick (and matching ball) is a dialectric, and one is a metal. They have exactly the same diffuse color. I have no doubt you can tell which is which.

The middle ball is the same dialectric material, but the diffuse color is black. I added that so you can see how reflections behave on a dialectric easily.

The table is also a dialectric, as are the candles.

When you look at a metal, almost 100% of what you see is colored reflections of the environment. That's why I think environment spheres (as I've used here) are so important.

If you are using a set of maps that describe a material that has both dialectric areas and metal areas, then the metallic map would be very important. For these objects, which are all of one material, I don't need a metallic map.

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Renderosity forum reply notifications are wonky. If I read a follow-up in a thread, but I don't myself reply, then notifications no longer happen AT ALL on that thread. So if I seem to be ignoring a question, that's why. (Updated September 23, 2019)