The ice diamond is one of the four Diamonds of Azzanadra which must be obtained during the Desert Treasure quest. It is obtained from the troll child by defeating Kamil and then freeing his parents from ice.
It is recommended that players deposit this gem on the bank as soon as they obtain it, along with the other Diamonds of Azzanadra, as a level 95 stranger may appear and attack them if they are holding one of them. The more diamonds you have on you, the more likely the stranger will appear and attack you.
Should the player lose the diamond or want to collect another via drop trick, speak to the troll child to obtain another. To keep a copy of the diamond after the quest, drop a duplicate before putting the last Diamond of Azzanadra into the pyramid's final unpowered obelisk and it will be safely kept afterwards, even upon quest completion.