Obstacles

Summary

Obstacles are plants 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.

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.
Vegetation respawns at a rate of one item per eight hours, except if all the spaces in your village are totally filled. Rocks however, do not respawn once removed.
Since vegetation respawns, and removing one may yield Gems, removing vegetation regularly can be an unlimited (albeit slow) source of Gems. This could be a good idea to farm Gems.

Your village can have a maximum of 40 obstacles at one time.

There is an achievement called "Nice and Tidy" which gives players Experience and Gems for removing 10/50/500 obstacles.

Troops can be spawned beside and on top of obstacles due to the new update.

Statistics

Vegetation

When the game respawns an obstacle, the respawn weight represents the chance of that particular item being the one that appears. Also, a new obstacle called the Gem Box was added, but quickly deleted due to a few glitches relating to how many gems it gives. They have been added back into the game on April 9, 2014, the update that added Clan Wars.

Type	Size	Removal Time	Experience Gained	Removal Cost	Respawn Weight
Mushroom	2x2	10s	3	100	10%
Bush	2x2	15s	3	250	20%
Trunk (1)	2x2	30s	5	1,000	10%
Trunk (2)	2x2	30s	5	1,000	10%
Trunk (fallen)	2x2	30s	5	1,000	20%
Tree (small)	2x2	30s	5	2,000	20%
Tree (medium)	3x3	45s	6	7,500	5%
Tree (large)	3x3	60s	7	10,000	5%

Rocks

Rocks, once removed, do not reappear.

Type	Size	Removal Time	Experience Gained	Removal Cost
Stone (small 1)	2x2	20s	4	500
Stone (small 2)	2x2	20s	4	500
Stone (small 3)	2x2	20s	4	500
Stone (small 4)	2x2	20s	4	500
Stone (pillar)	2x2	45s	6	10,000
Stone (large)	3x3	60s	7	20,000

Tombstones

Tombstones spawn when attacking Troops are killed.

Type	Size	Removal Time	Removal Benefit	Event at which appears
Tombstone	1x1	Instant	Gives 20 Elixir	Attacking or defending Elixir troop is killed.
Dark Tombstone	1x1	Instant	Gives 1 Dark Elixir	Attacking or defending Dark Elixir troop is killed.

Gem Box

The Gem Box is a mysterious box packed with gems! You can only have 1 gem box at any time in a village. Clearing the Gem Box costs 2,000 Elixir, takes 30 seconds to clear and will give you 25 gems.

Special Obstacles

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.

During the Winter 2013 Update, they introduced another X-mas Tree that gives 75,000 Gold when cleared.

Video of X-mas tree (2012) being chopped down and the reward: http://www.youtube.com/watch?v=3YC0GPHflwQ