Endometriomas are cysts that form when endometrial tissue grows in the ovaries. This tissue then responds to monthly changes in hormones. The tissue bleeds monthly, which may cause it to form a gradually growing cyst on the ovary. An endometrioma is also known as a ‘chocolate cyst’ because it is filled with dark, reddish-brown blood. An endometrioma is often linked to a condition known as endometriosis. It can be painful, especially during the menstrual period or during sexual intercourse.