There are several ways to buy gems and coins:
1. You can buy gems or coins by pressing the 2nd tab of the menu on Facebook ("Get Coins & Gems" ):
2. You can buy gems or coins by pressing on the "+" icons in the right corner above:
3. Sometimes a special offer will appear on the right side of the game and you will be able to purchase the gems/coins/other resources by pressing on it:
After you've chosen the package needed, you'll need to fill the payment method details and confirm the purchase.