Most often a regular jack, which is long (4 1/2″ to 6″) and thin (about 1/2″ wide) with a single spear point blade. They are used to cut a plug out of melons to display the ripeness of the fruit. Also used in the meat packing industry, most melon testers have a vendor’s name or advertising slogan inscribed on the handle. Some melon testers are double-ended, having a second pen blade or small fork. They can have a pocket clip. Do not confuse these inexpensive knives with “Fruit Knives.”
A double-ended Melon Tester.