What is the origin of a minaret, is it Christian and adopted later by Muslims in their mosques? Or is it a pure Islamic feature?
Minarets probably developed in the late 7th century Islam, adopted to the call to prayer (azan). The first minaret was built by Christian builders and later on minarets became a sign of power for Muslim rulers and they were used for prayer calls as well. So, minarets came into existence, but still there are many more details to be furnished and researched.
The Dictionary of Islamic Architecture, which is fully on ArchNet in the Digital Library > References section at http://archnet.org/library/dictionary/ has a long explanation on the evolution and history of minarets: