oil grit

Where does the grit collected by the oil filter come from?...What is grit?...Looks like fine sand. black ovals were larger
The air filter is new and clean. Yet after 3500 miles there was quite a bit of grit in the oil filter. not quite a 1/2 teaspoon.
Should I be concerned?
Thanks

The grit is carbon sludge, caused by the additives in your oil and some of the less resilient oil molecules burning up. Nothing to be concerned about as long as you change your oil every 3000 miles to avoid building too much of it. Ideally, you'd want to either get an engine flush product from a parts store or drop your oil pan every year to clean out the carbon sludge that has been building up.

If you want to reduce your production of carbon sludge, consider easing your car into a synthetic oil, starting with synthetic blend. If you have any oil leaks, make sure to fix them before starting to run synthetic, as it is a lower viscosity and will find any leaks you may have.

What carbon sludge does is it hangs out on the bottom of your oil pan and in your oil passages as a black, caked mud-like substance and adds unnecessary weight, and can even clog oil passages and add resistance to piston movement over time. When your engine gets hot, some of the carbon sludge melts and integrates with your oil, increasing the viscosity and friction coefficient of the oil and causing the engine to work harder. Cleaning it out every once-in-awhile will lead to better performance and gas mileage and longer engine life.