Black Diamond is a dark-skinned variety of watermelon that bears fruit weighing up to 50 pounds each. This variety produces melons with bright red flesh and a bruise-resistant rind. Thumping the melon to listen for a dull thud is not a reliable method of determining the ripeness of a Black Diamond. Monitoring the color and texture of the skin and the feel of the vines around the fruit is more reliable.
Calculate the age of the plant, starting with the day of planting. Black Diamond watermelons take 90 days to reach maturity, so if the plant is younger than that, the fruit is probably not ripe.
Feel the skin of the watermelon. When Black Diamond watermelons are ripe, the skin is rough. Try to pierce the skin with a fingernail. Ripe melon skin is too thick to pierce with a fingernail.
Monitor the color of the skin. As the Black Diamond matures, the skin turns dark green and becomes dull. The skin on the belly of the fruit, where it touches the ground, becomes a light yellow. The melon is ripe when it has a light yellow belly and dull, dark-green skin on the rest of the surface.
Pinch the vine tendrils near the stem end of the watermelon. When these tendrils are brown and dry to the touch, it's another sign of ripeness.