It appears that the product of any pair of twin primes (excluding the first pair 3 and 5) yields a semi prime whose digital root is equal to $8$. 

Example: $$ 17 \cdot 19 = 323 $$ The digital root of $323$ is $8$.

I've tested the first twenty and a bunch of random large ones such as $$ 8231 \cdot 8233 = 67765823 $$ Its digital root is also $8$.

As an amateur in advanced mathematics I'm curious to know how I could prove or disprove this conjecture. All tips are welcomed and much appreciated. Thanks.