Using a sharp knife, make a point at the end of each skewer. Carefully skewer the tuna lengthwise. Keep the tuna refrigerated while preparing the marinade. In the blender, combine the juices, honey, jalapeno(s), onion, cumin and coriander. Puree until smooth. Marinate the tuna for 15 to 20 minutes.
Combine the cucumber, mango, mint, sugar, salt, vinegar and canola oil. Stir to blend well; refrigerate for 30 minutes.
Heat a grill to medium-high and lightly brush the grates with oil. Remove the tuna from the marinade and grill for 1 or 2 minutes per side for rare and 5 to 10 minutes for well done.
Serve the tuna with the cucumber mango relish.