Hola. If you want to prevent a player from placing any amount of amethyst shards in a chest, you could use this code:
on inventory click:
if type of player's current inventory is chest inventory:
set {_item} to type of event-item
if {_item} = ("amethyst shard" parsed as an item):
cancel event
send "&cYou cannot place amethyst shards in this chest!"
The event triggers when someone clicks an item inside their inventory. First it checks if the inventory is a chest. Then it sets the variable {_item} to the type of item. This is important, because if it is just event-item it will also return the amount the player is clicking (e.g.: 64 amethyst shards). Then, if {_item} is indeed an amethyst shard, it cancels the event and sends the player a notice.
Hope this helped!