The most common and most likely answer to this crossword clue is the 6 letter word GEODES
I've seen this crossword clue in The Daily Telegraph
G
E
O
D
E
S
More clues leading to the result GEODES
Buddy explains!
The straight part
We know that "Stones lacking heart" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case stones lacking heart = geodes.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does GEODES mean?
GEODES noun- A nodule of stone having a cavity lined with mineral or crystal matter on the inside wall.