Download maybe welcomed by county, a gem! (8)
I believe the answer is:
sapphire
'a gem' is the definition.
(I know that sapphire is a type of gem)
'download maybe welcomed by county' is the wordplay.
'download maybe' becomes 'app' (an app might be downloaded).
'welcomed by' indicates putting letters inside (letters welcomed inside).
'county' becomes 'shire' (term for a county, eg Hampshire, Warwickshire etc).
'app' going inside 'shire' is 'SAPPHIRE'.
(Other definitions for sapphire that I've seen before include "Precious thing" , "A gem; I, perhaps (anag.)" , "Precious transparent stone of blue corundum" , "precious item" , "Bright blue" .)