Ceramic made from alumina poisonous?
Aluminium oxide is used in toothpaste (as an abrasive) and tablets (as an inactive filler). You'll get much more from these sources than from food preparation. 

It's essentially insoluble in water as well, making it hard to absorb from food. Some other aluminium compounds are much more soluble especially given that the stomach is acidic, and are a cause for more concern. 

For a (very) full review of the health science of aluminium compounds in general (including the oxide), see Human Health Risk Assessment for Aluminium, Aluminium Oxide, and Aluminium Hydroxide, 
Krewski et al. J Toxicol Environ Health B Crit Rev. 2007; 10(Suppl 1): 1–269.