I assume you know it shows Fort Denison. Although I obviously can't be sure from here, I feel this is likely to be a print. Emanuel had a huge output, including a very large number of sketches for his own books and for other works he illustrated. If it was an etching it would have an edition number and likely be signed in the margin. If it was an original art work, it would most likely be in pencil, or ink and watercolour. There are any number of his prints for sale on eBay for very high prices which are simply pages taken from one of his books. His works were also widely reproduced as calendars, portfolios of prints, even Christmas cards, often on the sort of thick paper you describe. As you seem to be able to take it from the frame, examine it closely with a strong magnifying glass - you should then be able to tell whether it is a mechanical print or an original. Sadly even original works by this fine artist struggle to make appropriate prices - I have several original etchings and one pencil work in my collection that I bought very reasonably. I feel he was simply too prolific and the type of architectural images he produced, often of public buildings, are not popular. Those works that do best show streetscapes, or buildings that may now be in private hands.
