Carve some ASCII jewels!
Charcoal, 17 bytes

Code:

ＮβＧ←β_↙↙¹→↘⁺β¹‖Ｍ→


Explanation:

Ｎβ                      # Place the input into β
   Ｇ←β_                 # Draw a line of length β with _ as the filling character
        ↙                # Move the cursor one down and one left
         ↙¹              # Draw a line from the cursor position to one position ↙
           →             # Move the cursor 1 to the right
             ⁺β¹         # Add one to the input and..
            ↘            # Create a line pointing ↘, with the size calculated above
                ‖Ｍ→     # Mirror to the right


A very neat command is ‖Ｍ, which also automatically mirrors / into \.

Uses the Charcoal encoding. 

Try it online!