Old hard form of silica surrounds geographical feature (10)
I believe the answer is:
historical
'old' is the definition.
'hard form of silica surrounds geographical feature' is the wordplay.
'hard' becomes 'h' (abbreviation used in pencil classifications).
'form of' is an anagram indicator (the letters need to be in another 'form').
'surrounds' indicates putting letters inside.
'geographical feature' becomes 'tor' (tor is a type of hill or mound).
'silica' anagrammed gives 'isical'.
'isical' enclosing 'tor' is 'istorical'.
'h'+'istorical'='HISTORICAL'
(Other definitions for historical that I've seen before include "Dealing with past" , "Relating to a past era" , "Based on records" , "Authentic" .)