Despite the delicious name, this mineral is not edible. It has the features of watermelons if you except the seeds, sweetness and softness. It is a variety of tourmaline. Images of the actual mineral can be seen here—a page that sells jewellery.
More of my images, including the nets, can be viewed in “Galleries” → “Papercraft” → “Crystals” → “Watermelon Stone”.
Next week I hope to introduce a mineral that allegedly glows in the dark.