I bought property in 1999 but just now receiving the mineral rights from previous owner. I want to sell them. Will the sell be considered short or long term capital gain?

I'm not a lawyer.
If you purchased the mineral rights in 1999, your should be entitled to treat this as a long-term capital gain.  However if you purchased the property in 1999 and purchased the property rights now, that would be a short-term gain.

Regards,  Don