Health Crystals are used for regeneration of your troops and spies, this is a Health Crystals only purpose, although it can be needed throughout the game for not wanting or needing to wait for them to regenerate fully to 100%.
Only 24 Health Crystals can be used within 24 hours. Once you have used 24 Health Crystals within a 24 hour period you will no longer be able to use the next until 24 hours past the time of the first Health Crystal you have used.
Being that most players use Health Crystals on epic battles suchs as Haunting the Escape, or other Premium epic battles, you are only allowed to use 6 Health Crystals max per epic battle.
With that being said... if you max out and use 6 Health Crystals in 4 straight epic battles, you will have reached your max limit and
will not be able to use another Health Crystal for 24 hours from the time you used the first Health Crystal.
iOS devices can purchase nobility points and Health Crystals through the Kingdoms at War App and iTunes.
Android devices can purchase nobility points and Health Crystals through the Kingdoms at War and Google play.
PC users can purchase nobility points and Health Crystals through Kingdoms at War with Paypal.