Amethyst clusters will not destroy properly when the source block it is on is broken, as long there is another block at the tip of the cluster. This will cause the cluster to be able to float mid air without a supporting block, and will only update if the source block is out back.

Steps to reproduce:

1. Place down an amethyst cluster on any block 
2. Place another block at the tip of the amethyst cluster
3. Break the original block the cluster is on
4. (optional) Break the block at the tip of the cluster

Expected result:

The amethyst cluster should break

Observed Result:

The amethyst cluster floats in the air without a supporting block