Unambitious
No, it is not (despite the LY ending). The word costly is an adjective (expensive, high-priced).
There are several reasons why a natural, gem-quality diamond is costly, the foremost being it's rarity. Only about 20% of all diamonds mined are gem-quality. Also, lab-created diamonds are less costly that natural diamonds. Industrial quality diamonds are not costly.
Dear,costly and high-priced are some of the synonyms for expensive.
costly
Lame
Slave.
Regression.
beleaguer
Unambitious
not funny
sry its fashionable
fault
The antonym for "happened" is "did not happen" or "did not occur."
flowing or continous
The antonym for will is won't, i think. won't or will not
unpleasant pressure, unhappiness