Onyx mine is obtained as a reward through the "Onyx Mine Tournament" that started 11/14/2014.
At Castle level 7 it is also available to purchase through the Onyx Market for the price of 360 Onyx.
"Fuel up this automated mine with essence to receive a steady supply of Onyx! Shuts tight when under attack to stop thieving raiders."
There is also a variant of the Onyx Mine that is "Supercharged" and produces 10 onyx in 4 hours. It is a temporary camp available only during promotions and alliance events.