It's quite easy to increase number of rubies you can get from Cooking Diary.  All you need to do is to visit friends' restaurants and collect their gifts. Their gifts are filled with rubies. Tap on the gift and collect it.  Get your friends to visit you as well. This way you can get more gifts and rubies.  Always remember to connect your Facebook account to get more rubies by winning smiles.  You can even get rubies by completing the alliance missions.