As you can see in this detailed part leak photo, the sensor is in the center of the metal disc, which would lie directly under the button.
I dunno. Looks pretty similar to a normal home button.
An RF sensor would normally have an array of metallic pixels (antennas).
The fingerprint processor is the adjacent square directly down the ribbon cable.
It looks like whatever goes on the square is missing.
The sapphire composition of the home button makes sense because it would be important to keep the integrity of the finger/sensor interface maintained and free of wear.
Sapphire makes no technical sense at all. For one thing, it's expensive to make and cut. More importantly, fingerprint sensors already use a carbon layer that's HARDER than sapphire. The only reason to use it would be to sound "cool".
The metal ring surrounding the home button is also key to this technology. Electrical contact with the users finger is needed for this RF imaging method to work.
The retaining ring around the button above, though, isn't that ring.
Interestingly, this metal ring could be constructed out of Apple's acquired LiquidMetal technology, which is reported to be transparent to RF. So this ring will NOT be an indicator light or anything.
LiquidMetal is NOT transparent to RF. It can conduct RF. Which is why it could used for a ring antenna, if there is one.
I still find it odd that the button had to be made convex in shape. A typical finger would be flush with the regular concave shaped button. Maybe it was necessary to fit the sensor correctly and have the exact tolerances of dielectric thickness.
As you said, convex would distort the finger and makes no sense.