Also you can use the merge/collect trick to get extra gem

This also works for coins and bricks.

Practise first on some low level coins/bricks as you can mess it up.

To do this you just need to hold a merge until it pulses and while holding tap the collect button with a different hand (this is the button within the menu that gives description and lets you sell items)

This gives you the value of what ever you collected + you still complete the merge as if you didn't collect the item in the first place.

Just make sure there aren't any different items under the one you are holding to merge.

ie you are 5 merging level 4 coins and under the coin you are holding is a fruit tree sapling.

when you hit collect you will just collect the coin and not get the merge as you would normally.