Ryan McLeod, Chase McBride, and Brice Tuttle created Gravity, an iPhone app which ingeniously used the 3D Touch display of the iPhone 6s to turn the device in a digital scale and weigh objects with an accuracy of ~1-3 grams. Alas, the app has been rejected from the App Store:
With the force values linearly correlated to weight, turning any force into a weight was going to be as simple as recording the force of known weights and creating a linear regression. It’d even be possible to use some statistics to predict how well the calibration went (there are many factors that can throw off a calibration). We opted to use coins for calibration, with a framework that made it easy to internationalize in the future.
To make a long story short the final answer over the phone was that the concept of a scale app was not appropriate for the App Store.
We were—and still are—bummed to say the least, but we understand some of the reasons Apple might not be allowing scale apps at this time.
I understand why Apple may not be sure about an app that requires placing a spoon on screen. Still, I hope that, eventually, novel uses of 3D Touch like Gravity will be accepted on the App Store. Make sure to read the technical details behind Gravity (and watch the video as well).
See also: weighing plums with 3D Touch.