Once it is knocked off, are the defence and special defence boosts still active?
No. The effects only last when the item is held
Source: Experience and see for yourself
https://replay.pokemonshowdown.com/gen8ou-1376089487-ne7yof8mxis2esl4a8wklwrcr3tqfc2pw
No it isn't.
When held, it raises the Defense and Sp. Def of a Pokémon that can still evolve.
Only when held, the boosts are active. the boosts aren't active when it is knocked off.
Source: https://pokemondb.net/item/eviolite
Hope this helps :)
Actually, the effect of the eviolite wears off if it is knocked off. The stats reset to normal after the knocking off of eviolite.
