The most common and most likely answer to this crossword clue is the 6 letter word OOLITE
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Buddy explains!
The straight part
We know that "Sort of stone" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case sort of stone = oolite.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 OOLITE mean?
OOLITE noun- A rock consisting of spherical grains within a mineral cortex accreted around a nucleus, often of quartz grains.
- An ooid or oolith.