Finally I have got my first Farmville Crop Mastery Sign for harvesting Grapes.
Farmville Crop Matery Sign is awarded after you complete 3 levels of mastery of any particular crop. The Crop Mastery Sign can be found in your Farmville Gift Box after you complete the 3 levels, from where you can place them on your farm.
You can show the Crop Mastery Sign at your farm and it tells everybody that, "Mastery Sign: Premium Grapes (+XP!) grown here".