You can use coins to buy gems in DragonVale, either from friends of from the market.
Visit A Friend’s Dragon Park
It might not seem like much, but free Gems and Money is too good to pass up. Visit a friend’s Dragon Park once every day and find the Party Hats on their island. The amount of money you receive depends on how big your own park is at the time of your visit, but doing so will help expand it faster than without gems.
With Dragons And Eggs
Babysitting your dragons is a good way to earn in-game cash to buy gems with. If you place your dragons on sale, you can earn cash and gems depending on how rare that dragon is.
Gifting With Friends
Every day, DragonVale allows you to send a single gem to three different friends. Remember to return the favor!
Trade With Friends
If you and a friend join together in DragonVale you can trade gems for Dragons and other supplies. Maybe you’ve got an extra dragon that they really want. Make sure to let them know you’d be willing to trade for some gems or another dragon in their possession.