The Lost Sunstones

"People hear of sunstones and they think it's a myth, but that just means they're not looking with their brains. Sunstones absolutely used to exist. We just lost the tech, the knowhow, to make them." 

— Explorer and Inventor Amit Depal

Utility

It seems these sunsontes were used in place of electricity, and that they were mostly used near/around/in water (which makes sense since electricity doesn't work while in water and nobody wants the hazard around).

Social Impact

This allowed light to be used in the water during night and/or deep drive, which in turn allowed a whole night culture to thrive when, without the sunstones, there was not much of a night culture.

Inventor(s)

The inventor is unknown, but assumed to be either someone living on the coast or living in the water (there is a heavier emphasis on the living in water part since talking about sunstones had lead researchers to believe that the items themselves were procured from the ocean, which is why it hard for people on land to recreate the objects.)

Access & Availability

It seems that these objects were a closely guarded secret, and when the people who knew about them died, the way of making them died as well.

Complexity

The complexity of the objects are unknown since the technology is missing/forgotten. However, most researchers believe the complexity to be higher than estimated (and the fragility) since there are none to be found and no one can figure out how such a small energy source was created in the first place.

Discovery

In Amit's case, the history of sunstones was discovered in a historical document. They were talked about as a tech item that powered small devices (usually the size of something that could fit in the hand palm). Another document found in the same place supposedly had a diagram explaining how these were made or used, however only a fraction of that page survived (enough to know there was a sketch on it). These "sunstones" either have a different name now (and have been since found), or cannot be found at all.