Some interesting facts regarding obstacles in coc which i think worth sharing.
-Each obstacle that is removed will yield 0-6 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 the 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.
-You can only have 1 gem box at any time in a village.
Source:
http://clashofclans.wikia.com/wiki/Obstacles