You need essences to complete collections. There are three main types of essences:

• Essence of Desert - common essence
• Essence of Life - rare essence
• Essence Of Starlight - epic essence, the rarest one

There are also 3 epic essences you will need to complete Rare collections - Tidal Stone, Luxurious Amethyst, Captivating Ruby.

You can find the amount of essence you need in the collection. You have two numbers separated by a backslash. The first number shows the amount of essence you have in total and the second shows the amount you need to complete the collection. If you have got enough essence the area will be green if not enough the area will be red.

You can get essences the following way:

• Complete match-3 levels. Lots of attempts may be required depending on how rare the essence is (yellow is a common one, blue essence is rare and you may need more attempts, purple is the rarest). If you do not have enough essence then just play levels in buildings, any type of essence can drop in any building.
• You can buy essences in the Shop
• Get epic essences for rare collections as reward for completing common collections

To buy essence, tap the Shop icon from the city to get inside. Switch 'Essences' tab. Choose the pack you need and tap the Buy button to get the essence.

You can also buy them by tapping the essence in the Collections.