Here, googled for you :
"A ceramic knife is a knife made out of very hard ceramic, often
zirconium dioxide. These knives stay sharp longer than steel
knives. They are also extremely brittle and will shatter or crack
when used to pry. They chip on the edge if used roughly. Usually,
though, the edge will merely chip when dropped. Although porcelain
is a type of ceramic material, unlike porcelain, ceramic knives do
not shatter easily.
Zirconia is very hard; it ranks 8.5 on the Mohs scale of mineral
hardness, compared to 6 to 6.5 for hardened steel, and 10 for
diamond. As such, they cannot be resharpened the same way as steel
blades, though they normally come with a special complimentary
resharpening service.
