Here is a more descriptive excerpt on the process taken from a write-up on the smart watch from Apple:

Unlike sapphire, which is artificially made in a process that requires extreme heat and pressure, ion-strengthened glass is glass that has been chemically altered via ion exchange to improve its strength. In this process, the sodium ions in the glass material are exchanged with larger potassium ions under high temperature. This is a cost-efficient process that produces a material more impact resistant and scratchproof than regular glass.

Ok more scratch proof than regular glass?  No biggie so is sapphire crystal.

In reading from this excerpt this may be the reason for the glass choice:

According to Apple Watch Sport page on Apple.com website, the Ion-X glass helps makes the Watch Sport lighter (inc. the 7000 series aluminum). Furthermore, it’s written that the glass is made using alumina-silicate glass. This glass is especially resistant to scratches and impact. So according to Apple it’s much more durable than regular glass, but what I understand from this that it’s not on par with Sapphire crystal as far as scratch-resistance is concerned.

So it seems like they made a choice for a notch above regular glass but still keeping it light.

thanks to Camera Debate for the awesome info

thanks to Kelly Hodgkins on Apple World Today for the great write-up