ModNote: Jump to Page 1 Article. --mkrishnan
In light of new evidence that show that the recent leaked iPhone 4G prototype are probably real (the debate now being more about if the released product will look like the prototype), I think that what's been identified by many as a glass back is really made of ceramics, like zirconia.
A patent application from 2006 reveals that Apple has been experimenting with ceramic enclosures for a while. The manufacturing process might have been a little more complex than expected, so that would be why Apple didn't release any ceramic enclosure before.
http://appft.uspto.gov/netacgi/nph-...28".PGNR.&OS=DN/20060268528&RS=DN/20060268528
https://www.macrumors.com/2006/11/30/radio-transparent-materials-in-iphone/
http://hrmpf.com/wordpress/102/radio-transparent-zirconia-iphone
Ceramics like zirconia have many advantages over plastic and metal for a device like a phone:
From the patent:
Usage of ceramics would explain why the back is not curved like previous iPhones. And while glass is also "radio transparent", it's much less durable and would be much more likely to break than ceramics if you dropped the device. I don't see Apple selling an iPhone with an enclosure almost entirely made of glass.
Edit: John Gruber seems to agree with me.
In light of new evidence that show that the recent leaked iPhone 4G prototype are probably real (the debate now being more about if the released product will look like the prototype), I think that what's been identified by many as a glass back is really made of ceramics, like zirconia.

A patent application from 2006 reveals that Apple has been experimenting with ceramic enclosures for a while. The manufacturing process might have been a little more complex than expected, so that would be why Apple didn't release any ceramic enclosure before.

http://appft.uspto.gov/netacgi/nph-...28".PGNR.&OS=DN/20060268528&RS=DN/20060268528
https://www.macrumors.com/2006/11/30/radio-transparent-materials-in-iphone/
http://hrmpf.com/wordpress/102/radio-transparent-zirconia-iphone
Ceramics like zirconia have many advantages over plastic and metal for a device like a phone:
From the patent:
Some of the reasons for using ceramics over other materials is that it is structurally strong, stiff and radio transparent. This is especially important for wireless hand held devices that include antennas internal to the enclosure. Radio transparency allows the wireless signals to pass through the enclosure and in some cases enhances these transmissions. Other reasons for using ceramics is that they are highly scratch resistant, have color embedded in it (no paint or coatings), can be made into a wide variety of colors, and provides a variety of surface finishes including smooth and rough. Furthermore, the density of ceramics is typically higher than other materials therefore their weight is higher for the same sized part. This additional weight makes the handheld device feel more robust and it makes the device exude greater quality.
Usage of ceramics would explain why the back is not curved like previous iPhones. And while glass is also "radio transparent", it's much less durable and would be much more likely to break than ceramics if you dropped the device. I don't see Apple selling an iPhone with an enclosure almost entirely made of glass.
Edit: John Gruber seems to agree with me.