The designers aimed to keep the front of the phone as simple as possible while making the back distinctive and recognisable with a faceted form.
Here's some text from One & Co:
One & Co announces a new collaboration with HTC on the Touch Diamond, an innovation in both technology and design.
The HTC Touch Diamond, a collaborative effort between the HTC design team in Taiwan and the One & Co design team in San Francisco, is an iconic design that relies neither on minimalism or overt ornamentation. The design celebrates the rich, innovative user interface using a disciplined, geometric formal language to ensure a clutter-free environment for viewing content.
With the innovative design of the rear housing, the Touch Diamond is instantly recognizable and familiar. Drawing from a variety of different inspirations, the irregular back facets are juxtaposed against the front surface simplicity.
The collaboration spared no effort to create a product that is perfect - from design, to engineering, and manufacturing. Hundreds of materials and finishes were explored to find the perfect stainless perimeter and the high quality, precision manufacturing.
HTC has traditionally designed advanced, sophisticated smart phones for business users or early adopters. With the rapidly expanding smart phone market, HTC is responding to the demand to take the internet mobile for the consumer - expanding the services to include advanced, fun communication and entertainment. The design of the Touch Diamond reflects this new target and establishes HTC as a leader in the marketplace.