A portal in marble, for example (5)

I believe the answer is:

agate

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

'marble for example' is the definition.
'agate' can be an answer for 'marble' (thesaurus). I am unsure of the 'for example' bit.

'a portal' is the wordplay.
'portal' becomes 'gate' (I've seen this before).
'a'+'gate'='AGATE'

'in' is the link.

Can you help me to learn more?

(Other definitions for agate that I've seen before include "Semi-precious quartz" , "Semi-precious stone" , "marble" , "Quartz variety" , "Ornamental form of quartz" .)

I've seen this clue in The Telegraph.

Want a hint initially instead of a full solution? Install my app