Crossword Genius

Stone some used to flag a terrace (5)

Ross

I believe the answer is:

agate

I'm a little stuck... Click here to teach me more about this clue!

'stone' is the definition.
(agate is a type of stone)

'some used to flag a terrace' is the wordplay.
'AGATE' is hidden in amongst the letters.
But, I am not sure how this is indicated.

Can you help me to learn more?

(Other definitions for agate that I've seen before include "Semi-precious quartz" , "Banded ornamental stone" , "Type of quartz" , "Hard stone with concentric bands of colour" , "Quartz with bands of colour, used in jewellery" .)

I've seen this clue in The Globe and Mail.
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