Sprailroad said "Need the cannon ball guy on this."
I wasn't going to comment, because I don't know what the ball is, but since you've requested my input...
The "non-metallic" ball's metric size translates to 1.49-inches. That's too small to be a Colonial Era (or later) cannonball. Could it be a VERY-early stone cannonball? Maaaybe... but what stone ball found on a European beach can't be a stone cannonball? The calibers (Bore- Diameters) of the earliest (pre-Colonial-era, 1300s/1400s) European cannons are not known.
Based on the non-metallic ball's size and weight, in comparison to a same-size ball of cast iron, the non-metallic ball's Specific Gravity is a little bit above 3.0, and most minerals/stones which are not metal ores have a Specific Gravity below 3.0. For example, the SG of granite is 2.66, and limestone (common in the UK) is 2.65, clay is 2.51, quartz is 2.65, and cement is 2.67.
The sparking could be caused when a grinder hits a grain of a "sparking mineral" such as quartz or flint (both of which are Silica).
When I first read the finder's post and saw the photos, I thought the ball might be an extremely "Graphitized" iron ball. Graphitized means the iron object has lost much of its "surface/exterior" iron molecules due to tannic acid exposure in a swamp or coastal marsh. The tannic acid chemically leaches out the iron molecules, leaving behind the naturally-occurring Graphite (an isotope of Carbon) in the iron object. That could explain what the finder describes as a "metallic smell." Graphitized iron has a silvery look when you make a scrape-mark on it. Lastly, an EXTREMELY graphitized iron object will weigh significantly less than it originally weighed, due to the extreme loss of iron molecules from within it. Sounds on-target so far, regarding this ball. But... Graphitized iron is very soft (Graphite is the mineral in pencil-lead), so a grinder would have eaten into it very rapidly, and graphite won't spark.
The photo below shows a Graphitized cast-iron civil war artillery shell, found in a marsh. Note the color of the Graphitized iron. It came out of the marsh looking just as you see it in the photo. NO rust. The surface of Firehelm100's "non-metallic" ball has a similar look.
You asked for my input. The paragaphs above are all I've got. Maybe some of it will put somebody else onto the right track. I don't know what this non-metallic ball is.
