Interesting discussion about the creation of smart rocks and smart pebbles, which allow the tracking of current. These contain a data logger embedded in either aluminum or plastic. The tricky bit is how one finds them after they have been dropped off. Some ways might be radio transmitters, metal detectors and so forth but all of these have downsides.
How to Build a Smarter #Rock? Put a data #logger into aluminum or plastic; then track via radio or metal detector
http://www.sciencemag.org/content/338/6113/1412.summary
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