Corundum is nothing more than aluminum oxide. Adding the correct impurities, man has learned to synthetically grow beautiful gems like star saphires. The melt furnaces that are used to create aluminum castings also have the correct conditions to synthetically grow corundum crystals on their walls. Poor cleaning practices dislodge these crystals into the melted aluminum. They remain suspended because the density of aluminum oxide is very close to that of molten aluminum.
Many prospectors still search the globe looking for corundum gems like rubies and saphires. People who machine aluminum castings have automated the process of discovering corundum crystals. A discovery event occurs when the machining carbide is chipped. Carbide is only 8.5 on the Mohs scale which is less than the 9 Moh number for Corundum. Most machining processors that I know are very unhappy with discovery events.

Since aluminum die castings are usually made of material that is 95% recycled, they contain corundum crystals within the allowed 5% of non-metallic impurities. Most likely these crystals were a part of the material that was recycled and were not introduced by poor practice by the current casting producer.

The real question is what should be done to reduce the frequency of chipping carbide during machining. The simple answer is to use primary aluminum as the casting material. Usually not attractive because it costs twice as much as secondary (recycled) aluminum. More effective is to design a better match between the casting and the machined part. For example the .030 (.75mm) skin is free of oxide impurities. Accomplishing a machining reduction is always an effective way to reduce carbide chipping but this requires a higher skill. Certain parts like fan clutches, that are mostly machined, including deep grooves made using expensive cutters, are in another category. Purification processes such as Wedron that separate out the imprurities are needed. This increases material cost because someone must pay for buying the material that was removed and scrapped.