Hi,
One way might be to just let the mud dry and see if it shrinks and cracks.
Then it is much easier to remove it from the crevices with water.
Then let dry again and repeat etc.
If it shrinks, the mud is made of clay minerals.
If not, maybe the mud contains some calcite, then the removal is simple with diluted acid. Even if not all of it dissolves, the mud will become crumbly and more easy to remove.
Do not immediately immerse the crystal in acid though, because you do not know what happens inside the crystal and once the acid is in there it takes a long time till it is completely removed again.
Instead, first apply the acid externally only and check what happens.
This takes a lot of patience.
However, I would not remove all of the mud.
It depends on the crystal, but removing all mud may ruin the specimen.
The crystal might look very boring or its skeletal structure might not be that obvious anymore.
No one in his right mind would completely clean skeleton quartz from Poretta Terme, for example.
The mud makes for a stark contrast with the very shiny crystal faces and this looks very aesthetic.
It's a matter of taste, of course.
Just be aware that once the mud is removed, it cannot be put back into the crystal.
Amir
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