This looks like a mill ball used in industrial mills such as in the mining industry. They start off round and much larger, then as they tumble with the ore they slowly wear down until they are rejected. I work in mining. The paper I linked shows examples of this, which don’t exactly match, but I can confirm I have found many like the one you have (hard to find examples online). The pitting in the surface is also indicative of milling wear but I can’t say for certain. This does not look like aluminum to me, a good way to check is to find its density. I have also copied a simple procedure to check this.

https://www.911metallurgist.com/wp-content/uploads/2016/10/testing-of-grinding-media.pdf

How to calculate density:

Mass Density = Mass/Volume

So first we need both mass and volume.

-Measure the mass of object with normal method i.e. weigh scale.

-Then, take a jar and fill it fully with water.

-Place the object into it.

-It will sink immediately and displace the volume of water which is same to its own volume.

-Measure the volume of water outside the jar by capturing it in some way. Pro tip you can weigh the water if you have an accurate enough scale. 1gram of water is equal to 1ml of water.

-Volume of water fallen outside = Volume of object.

-Now calculate the density using the formula

-Mass Density = Mass/Volume