In Omni 1:20 we read:
And it came to pass in the days of Mosiah, there was a large stone brought unto him with engravings on it; and he did interpret the engravings by the gift and power of God.
Where would Joseph Smith get the idea of writing histories on large stones? The large stone in Omni is entirely consistent with ancient stone monuments called stelae discovered throughout Mesoamerica.
Evidence Central notes how the large stone in Omni shares characteristics with stelae:
Is this simply a coincidence?