Obstacles are trees, logs and rocks that are randomly placed in your village. You start with about 40 obstacles around your village. You can remove them if you have an available Builder and a small amount of Gold or Elixir. Gold is used to remove rocks and Elixir to remove vegetation.
Stone (Small 1)
Stone (Small 2)
Stone (Small 3)
Stone (Small 4)
Halloween Decorated Obstacles
Halloween Mushroom (2012)
Halloween Mushroom (2013)
Halloween Trunk (1)
Halloween Trunk (2)
Halloween Trunk (Fallen)
Halloween Tree (Medium)
Halloween Tree (Large)
Halloween Headstone (2014)
Winter Decorated Obstacles
Winter Trunk (1)
Winter Trunk (2)
Winter Trunk (Fallen)
Winter Tree (Small)
Winter Tree (Medium)
Winter Tree (Large 2012)
Winter Tree (Large 2013)
X-Mas Tree (2012)
X-Mas Tree (2013)
X-Mas Tree (2014)
Each obstacle that is removed will yield 0, 1, 2, 3, 4, 5, or 6 Gems. The distribution is not random and follows the following repeating sequence: 6, 0, 4, 5, 1, 3, 2, 0, 0, 5, 1, 0, 3, 4, 0, 0, 5, 0, 1, 0. This means on average, each obstacle cleared yields 2 Gems. This pattern does not include 25 gems yielded from the Gem Box, or the holiday themed obstacles like the Halloween Headstone (2014) which yields Elixir; the X-mas Tree (2012, 2013, and 2014) yields Gold instead.
Vegetation re-spawns at a rate of one item per eight hours, except if all the spaces in your village are totally filled. Furthermore, an obstacle will only spawn if there is a 1-tile buffer zone between the spaces in which it will spawn and another building or obstacle.
Rocks do not re-spawn once they are removed.
You can't place buildings on obstacles.
Since vegetation re-spawns, and removing one may yield Gems, removing vegetation regularly can be an unlimited (although slow) source of Gems.
Your village can have a maximum of 40 obstacles at one time.
When the Gem Box was added it was quickly deleted due to a few glitches relating to how many Gems it gives. The Gem Box was added back into the game on April 9, 2014, the update that added Clan Wars.
You can only have 1 gem box at any time in a village. Clearing the Gem Box costs 1,000 Elixir, takes 30 seconds to clear and will give you 25 gems. The gem box is rare but in a recent update they made them more common.
Note that all special obstacles (and the Gem Box) ignore the one-tile buffer zone rule that governs where normal vegetation can spawn. They can spawn in any 2x2 empty space within your village, though they tend toward spawning close to the outer edges of the field.
During the Winter 2012 Update, they added a X-Mas Tree that gives 30,000 Gold when cleared, which was then increased to 50,000. On 3 January 2013, Supercell indicated on the forums that people will get to keep their trees even after the winter theme is gone. The 2012 X-Mas Tree appears as a small tree with flashing colorful ornaments.
There's a 2012 edition of X-Mas Tree in the far left edge of the map in Gold Rush mission in Single Player Campaign in the line of trees that are surrounding the map.
During the Winter 2013 Update, they introduced another X-Mas Tree that gives 75,000 Gold when cleared. The 2013 X-Mas Tree appears as a small tree with golden garlands, a golden spikeball on top, and two skulls at the bottom of the tree.**During the Winter 2014 Update, Supercell introduced an X-Mas tree that not only gives 75,000 Gold when cleared, but randomly spawns wrapped presents worth 5,000 Elixir each when cleared. The 2014 tree is similar looking to the 2012 one, but is adorned with tinsel, candy canes as well as colored ornaments and flashing lights.
There's a special kind of Army Camp in the Goblin Gauntlet mission in Single Player Campaign that only consists of a camp fire with different surroundings that the one in the Army Camp, which is only 2x2. You can notice this by the other obstacles that are next to the camp fire. You can also drop troops over it since it is considered as an obstacle.
The rainbow effect on the Gem Box might be a reference to the saying "There's a pot of gold at the end of the rainbow".
If no gems are purchased, earned through achievements or spent on various purposes, 1623 obstacles (not including gem boxes) are needed to clear to buy the 3rd, 4th and 5th builder huts which cost 3500 gems in total, which roughly translates to 528 days (counting from the first obstacle removed and assuming that the village always contain less than 40 obstacles, in order for new obstacles to spawn).