Best article on unknown stone (5)
I believe the answer is:
topaz
'stone' is the definition.
(I've seen this before)
'best article on unknown' is the wordplay.
'best' becomes 'top' ('top' can be similar in meaning to 'best').
'article' becomes 'a' (indefinite article).
'on' says to put letters next to each other.
'unknown' becomes 'z' (algebra).
'top'+'a'+'z'='TOPAZ'
(Other definitions for topaz that I've seen before include "Shade of yellow -- gem" , "Yellow quartz used for gemstones" , "South American hummingbird" , "Yellow or pale blue gem" , "Shade of dark yellow" .)