Countertop material for the island doesn't have to match the rest of your kitchen countertops as long as it is harmonious with the room's overall design. You may want to splurge on solid surfacing here, for example, and use laminate on the other countertops. A butcher-block countertop is ideal for chopping, while granite or marble works well for baking purposes and for rolling pastry dough.
If you are looking to do a complete kitchen remodeling for yourself, or for maximum resale value, don't neglect the kitchen island. Once just a place to pull up some stools so the kids could eat before running to the bus, the island and it's function in the kitchen has changed quite a bit in the last 10 years.
Another problem you might face if you buy your kitchen island as a separate unit, and not as part of the overall kitchen, is matching it to your existing cabinets. This might not be as easy as your think, but if you purchase a solid color, such as black, white or red, then you should get away with it.
Islands can "float" as well. Smaller islands used primarily as an aid in cooking or baking can be fitted with wheels and moved around the kitchen as needed to aid in cooking. Whether you consider a floating island will probably depend on what you have installed on the floor now, or plan to install (a soft wood will be nicked and scratched).
kitchen island chairs
kitchen island fireplace
kitchen island with seating