Ice and diamonds are both transparent solids. Diamond has a higher index of refraction, which allows it (when properly cut) to glitter more impressively than ice does.
Uncut unshaped diamonds look like shiny black coal
Black diamonds are diamonds, but they are black, and not transparent. Follow the link, below, to see photos of black diamonds.
Yes, ice is a term sometimes used to describe diamonds.
Faceted, finished brown diamonds are simply diamonds within that colour range. Chocolate diamonds are an intense brown colour.
Out of the ground they look like little rocks/crystals. It is only when they are cut and polished that they look like they do in a ring.
You can learn more about diamonds and their visual dazzle, below.
They look like cyan (light-blue) crystals of some sort.
Ice caps look like towers of ice.
In their natural state, diamonds look like transparent crystals with a shiny, reflective surface. They are typically colorless or have a slight yellow or brown tint.
Jewelery can contain real diamonds, and jewelery can contain other gems, including stones meant to look like diamonds.
When describing diamonds, glass is probably not as common as ice.
They are only in the 1.3.1 update and you have to look for them like you have to look for diamonds :P