chemical inertness
Why line a furnace, a crucible, or a chemical reactor with ceramic? Because ceramics mostly refuse to react. Chemical inertness is a ceramic's resistance to being attacked, dissolved, or corroded by its surroundings: acids, alkalis, molten metals, hot gases, body fluids. A porcelain dish shrugs off lemon juice; an alumina tube carries a red-hot corrosive gas that would eat steel.
The reason is again the bonding. Many ceramics, especially the oxides, are already the 'burnt', fully-reacted, lowest-energy form of their elements: alumina is aluminium that has already combined with oxygen, so there is little chemical energy left to be gained by reacting further. (This is why metals corrode toward oxides while ceramics mostly do not: the ceramic is the destination.) The strong ionic-covalent bonds also hold every atom tightly, giving a would-be attacker no loose electrons and no easy foothold. Rate matters too: even a reaction that is favourable can be so slow at room temperature as to be irrelevant.
Inertness is a range, not an absolute. Silica glass dissolves slowly in hydrofluoric acid and in hot strong alkali; some nitrides and carbides oxidise (slowly growing a protective oxide skin) in hot air; certain glasses leach ions into water. So engineers speak of chemical durability under specified conditions. Where inertness holds it is priceless: acid-proof tiles, crucibles for molten metal, spark-plug insulators bathed in burning fuel, and biocompatible implants (alumina hips, zirconia crowns) that sit in the body for decades without dissolving.
Chemists heat reactions in an alumina or silica crucible rather than a metal one because the ceramic does not join in: it withstands molten salts and corrosive fumes at 1500 degrees C that would dissolve or oxidise almost any metal container.
Ceramics are often the already-oxidised, lowest-energy form, so there is little left to react.
'Inert' is condition-dependent, not absolute: silica is attacked by hydrofluoric acid, and many non-oxide ceramics slowly oxidise in hot air. Always state the environment.