It is normal for there to be veins visible underneath the tongue, partly because the mucous membrane is so thin and translucent in this region, but where these vessels become dilated and tortuous, they may appear round and black like caviar.[2] Caviar tongue is also referred to as sublingual varices (plural) and varix (singular) and look like varicose veins in the tongue. It is a benign, asymptomatic, venous lesion.
^James, William D.; Elston, Dirk; Treat, James R.; Rosenbach, Misha A.; Neuhaus, Isaac (2020). "34. Disorders of the mucous membranes". Andrews' Diseases of the Skin: Clinical Dermatology (13th ed.). Edinburgh: Elsevier. pp. 800–801. ISBN978-0-323-54753-6.