Shards and Stones in Reward Zone do not remain available indefinitely. Shards expire one year after they are credited to your account, and each Stone has its own expiration date displayed in the ZEN.COM app.
When Shards expire
Each Shard, including Gratitude Shards and Synergy Shards, expires one year after it is credited to your account.
After expiration, the Shard is automatically removed from your account and cannot be used to obtain Stones.
When Stones expire
Each Stone has its own expiration date. You can check this in the Stone details within the ZEN.COM app.
The expiration period begins on the date the Stone is credited to your account. After the expiration date, the Stone expires and cannot be used.
Related article: Can Shards or Stones granted in Reward Zone be reversed?