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For a great number of years, I was a mason and chimney sweep before rolling out to full bathroom and ktichen remodeling. It still is my first love. Please have a certified, licenced chimney sweep out to look at the chimney. /http://www.ncsg.org/ can put you in contact with a certified chimney professional in your area. Please do not use it until it is checked.
Hard to say based on the limited description available. Is the fireplace constructed completely of masonry? or is there an insert in the firebox? Is it framed walls with stone/brick veneer? Is the chimney a metal pipe or clay flu?
If you haven't used it and it is cracking, I would suggest that the foundation was possibly inadequate and it is settling.
Or the materials used were inferior materials, or the mortar wasn't a quality mix.
There are many factors that could be the cause. Hard to say based on the limited information.
I will say that converting from a wood fireplace to a gas fireplace shouldn't be much of an issue. Converting from gas to wood on the other hand is virtually impossible without major remodeling and expense.
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