When I looked at Nokia's 7500 Prism I was not overly impressed. That mobile had an original design constructed to make it look like a gemstone. As long as you were looking at the phone in pictures, for example on the web, the effect worked well. But the plasticky build of the handset in real life was just one of its let-downs.
Built in the same style, the quad-band 3G 7900 Prism is also primarily made of plastic though the casing is tougher and the back cover is made of aluminium. These factors help raise the overall weight from the 7500's mere 83g to 101g and as a consequence the 7900 Prism feels like a more substantial phone in the hand.
Size-wise the two mobiles are comparable with this handset's 112mm x 45mm x 11mm dimensions making the 7900 slightly taller and wider than the 7500, but thinner. The odd ‘diamond cut' design is retained in the 7900 Prism and is used both front and back.
Lighting is a big feature of this mobile. In one configuration the number pad is completely devoid of markings when the handset is in standby mode. When you press a button the numbers acquire their backlight. The colour is up to you. There are 49 to choose from, and making a selection is easily done via the Settings area.
Alternatively you can set up lighting effects. These include having lights coming into play when the phone goes into standby mode. At this point the screen is darkened and just the 4, 5 and 6 keys are backlit.
Meanwhile lights on the top edge pulse slowly in and out to show that the phone is active. Their colour is affected by whatever you've chosen for the keypad. I can't see the point of them to be honest. They cover a relatively small area, aren't visible when the phone is flat on your desk, and inevitably drain power.