The gem is on the second floor of the barn behind a haystack. To reach it, use a grenade or, if coming back from later levels in the campaign, your Signal Pistol. You could also use Jetpack to get up there.
It has a red barn with hay block piles and a tractor. It also has a farm house with 2 holes in the wall, a wheat field, a blue silo, a see-saw, a swing, and a combine that is not working and a forest surrounding it all. The bulldozer has the word DOG on it, referencing CAT as in real life.
Do not get too close, as the zombies will hit you.
Kill the monsters as quickly as possible, since you earn a star by killing monsters quickly. Do not rush it, though.
Use pets, as a minor/major backup for killing.
Follow the green arrow that appears once there are 5 enemies left, in order to kill the enemies quicker.
The enemies are rather weak in this level, so you can equip the weaker weapons.
Restore your ammunition by quitting the level and coming back. However, try not to do this when you are about to complete the level, as the amount of killed monsters will be wasted.
Use Armor. If you have a decent amount of armor, you will not have to worry about getting damaged, because you can get 3 stars even if you have damaged armor. This doesn't apply to Health.
The combine in this level has "DOG" written on it. This may possibly be a reference to the machinery-designing company "CATERPILLAR INC." which is simply abbreviated as "CAT".
Even though the assault from the zombies had just started, the hero's house already has two walls broken down and his entire second floor is filled with debris. It might be implied that the Survivor barricaded the second floor, but it is still unknown how there is 2 holes in the house.
It is heavily implied that the hero was a farmer before the events in Pixel Gun 3D, so it is unknown why he would forget to turn off both his tractor and combine harvester before going to sleep.
In the 9.4.0. update, the combine has been turned off and won't kill players.
Looking over the map, it seems that this is the only farm around.
There is a hole dug in the front that you can fall into. It used to kill you, but this is no longer possible.
In older versions, the farmhouse had a different interior design, most notably the mass amount of bloodstains on the walls and floors and the lack of furniture compared to recent versions.
In the living room, there is a picture, which possibly represents Marilyn Monroe in 4 different fillers.