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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.
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.
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