While many crystals can be safely placed in salt, some should be avoided as they might be sensitive or reactive to salt. Examples of crystals that should not be placed in salt include:
Selenite
Lepidolite
Kyanite
Calcite
Azurite
Malachite
It's essential to do your research or consult a knowledgeable source before placing any crystal in salt to ensure that it's safe and won't be damaged.