I've come across a particular situation where imps refuse to mine gems. I've made a small demo map that clearly shows the issue.
To reproduce simply load the attached map, select the gems to mine and notice the gems will never be mined: Testmap759.zip
The map shows a single block of gems on an island, rocks on 3 sides, hero ground on another. If you change the map to replace the hero ground with water, the gems will be mined.
Found in multiple versions, including 0.4.6 main release and version 0.4.6.1821.