Transcending the title, which actually detracts from the essence of the question, the connotation of artificial in the adjective contrived recommends it highly:
1.0 Deliberately created rather than arising naturally or spontaneously.
1.1 Created or arranged in a way that seems artificial and unrealistic:
The verbal root of contrived adds perspective:
1. Create or bring about (an object or a situation) by deliberate use of skill and artifice:
The antonym would be genuine:
1.0 Truly what something is said to be; authentic:
The etymology of these words supports their use for this purpose:
early 14c., from Old French controver (Modern French controuver) "to
find out, contrive, imagine,"
from Late Latin contropare "to compare"
(via a figure of speech),
from Latin com- "with" (see com-) + tropus
"song, musical mode,"
from Greek tropos "figure of speech"
1530s, from Latin tropus "a figure of speech," from Greek tropos "a
turn, direction, course, way; manner, fashion," in rhetoric, "turn or
figure of speech," related to trope "a turning" and trepein "to turn,"
from PIE root trep- (2) "to turn" (cognates: Sanskrit trapate "is
ashamed, confused," properly "turns away in shame;" Latin trepit "he
turns"). Technically, in rhetoric, "a figure of speech which consists
in the use of a word or phrase in a sense other than that which is
proper to it" [OED], "as when we call a stupid fellow an ass, or a
shrewd man a fox" [Century Dictionary].
1590s, "natural, not acquired,"
from Latin genuinus "native, natural,
from root of gignere "beget"
(plural genera), 1550s as a term of logic, "kind or class of things"
(biological sense dates from c. 1600),
from Latin genus (genitive
generis) "race, stock, kind; family, birth, descent, origin,"
root *gene- "to produce, give birth, beget," with derivatives
referring to family and tribal groups.
In the high-fantasy context, genuine objects are born metaphorically into the plane of existence and remain naturally because they are part of its essence, while contrived objects are manufactured in the plane of existence and must be maintained because they are not essential.