I possess many silver bars and sufficient amount of sapphires. I still can't see the desired item in the crafting list.
According to the wiki:
1.2.3: Damaged decreased and mana use increased.
This means staff was introduced in the 1.2 update to PC version of the game.
The 1.2 update was ported over to mobile version of the game in December 2014. You need to update your game to a minimum of 1.2.6508 on iOS, 1.2.6667 on Android (from Google Play - the Amazon app store version has not received the update as of writing) and 1.2.0 on the Windows Phone.
See the mobile version history article on Terraria wiki for more information.