On the Words of Marco (one of the founders of the project) openQCM:
openQCM is the first open hardware Quartz Crystal Microbalance (QCM) with applications in a wide range of scientific fields,
such as chemical and biological sensing, material science.
The sensing core is a piezoelectric quartz crystal oscillator. The deposition on the quartz surface of a very tiny mass (down to 1 billionth of gram)
causes the variation in the quartz frequency.
It has an Arduino board plus a custom electronic for driving the quartz resonator.
The hardware case is made using 3D-printing technology.
openQCM was built keeping in mind the emergent principles of the open source hardware movement,
so that anyone (scientists, technology enthusiast, makers, hobbyist …) can study, modify, and develop the hardware based on the original design
Check the working principle of the microbalance on the video below! (from: https://www.youtube.com/watch?v=kzmBK9mONq8 )