conjecture vs conjectural

conjecture is a noun but conjectural is not a noun.

Word	Noun	Adjective	Verb	Adverb
conjecture	Yes	No	Yes	No
conjectural	No	Yes	No	No

conjecture (noun)	conjectural (noun)
a hypothesis that has been formed by speculating or conjecturing (usually with little hard evidence)
reasoning that involves the formation of conclusions from incomplete evidence
a message expressing an opinion based on incomplete evidence

conjecture (adjective)	conjectural (adjective)
	based primarily on surmise rather than adequate evidence

conjecture (verb)	conjectural (verb)
to believe especially on uncertain or tentative grounds