Shards and Stones in Reward Zone do not stay available forever. Shards expire one year after they are granted to your account, and each Stone has its own expiration date shown in the ZEN.COM app.
When Shards expire
Each Shard, including Gratitude Shards and Synergy Shards, expires one year after it is granted to your account.
After it expires, the Shard is automatically removed from your account and cannot be used to get Stones.
When Stones expire
Each Stone has its own expiration date. You can check it in the Stone details in the ZEN.COM app.
The expiration period starts on the date the Stone is granted 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?