More clues are out that indicate sapphire is a material that will play a crucial role in the rumoured iPhone 6 release date, which likely will happen in the second half of 2014.

In an interview with U.S. TV network ABC, Apple CEO Tim Cook has admitted that his company's recent investment with a sapphire-making firm involves sapphire glass. Prior to this, the next iPhone is touted to sport sapphire glass with the phone's casing blanketed in the same material.

Yet according to BGR, Cook quickly clarified that the sapphire glass Apple is developing should not be automatically connected with the upcoming iPhone model this year, which latest reports said will come in two screen sizes - one of them a phablet-size at 5-inch.

When pressed to elaborate, Cook dodged the question by insisting that the matter cannot be discussed at the moment.

And as the Apple chief provided hints that sapphire will figure more in the iPhone 6 make than in previous models, reports from Asia suggested that the tech giant is indeed serious about a smartphone that is totally wrapped in sapphire.

The same material, according to Apple Insider, is protecting fully-functioning iPhone prototypes that Apple manufacturing partner Foxconn is currently testing.

"Apple manufacturing partner Foxconn is said to have successfully assembled at least 100 prototype next-generation iPhone units with sapphire-covered displays," the tech site said on its report, adding that the development suggests that sapphire-covered iPhones are now ready for volume production.

While the shift from Corning Glass to sapphire demanded more complicated manufacturing techniques, Foxconn's assembled prototypes proved that sapphire in the iPhone 6 is wholly viable, indicating too that mass production will be able to support bulk shipment requirements by Apple prior to the device's release date.

It should be noted too that Foxconn's subcontractors, firms that were involved in producing the sapphire coatings used in the iPhone 5S rear camera lens and its TouchID fingerprint, were part of the project, likely contributing in expediting completion of the manufacturing experiment.

In an earlier report, the iPhone 6 appears headed to sport supersized screen that is protected by sapphire. The same is also tipped as Apple's preferred material to protect not only the next iPhone's display panel but also its full body.

On release date, the iPhone 6 is whispered too to show off a 64-bit A8 processing chip, will run on iOS 8 will all the high-end inner components to be shielded by a virtually indestructible Liquidmetal body frame.