I know having it set horizontally mimics an eye opening and closing in its layout but having it set vertically allows you to move your hand sideways for better fun effects, not just the eye opening but for the milk and honey effect (plus having a wicked cool snake eye’s slit)
Am I really the only one who voted for horizontal? I like the way the reflection in the stone mimics the reflections in the gold and creates a smooth line instead of perpendicular contrast. Both are beautiful but I'm not a fan of the snake eye look.
But, it really depends on the stone. Phenomenon stones often look more interesting from a particular direction. I have a teardrop Oregon sunstone cab that I wear "upside down" because I prefer watching the schiller from that direction.