Simply a finger ring with built-in photoplethysmographic measurement display capability.
Like a mood ring, but with either numeric, scalar (thermometer-like) or color display of heart rate.
The basic technology has been available since the early 1970s [link.] I built a version of the referenced
device when I was about 14.
Possible applications: Medical, entertainment, romance, psychotherapy, ...
[Other non-invasive physiological measurements encorporated by reference: pulmonary function, blood pressure, sexual arousal, perspiration, et al.]