Endometriosis lesions come in a surprisingly wide range of colors and forms, which is one reason they’re often missed during surgery. The classic description is a dark “powder-burn” or “gunshot” spot that looks black, dark brown, or bluish. But more than half of all lesions are actually subtler in color, appearing red, clear, or white, and these are the ones most likely to go unrecognized.
The Color Spectrum of Lesions
When a surgeon looks inside the pelvis during laparoscopy, endometriosis lesions fall into a few broad color categories. In a study of 133 women with surgically confirmed disease, 30% of lesions were red, 28% were black, 23% were white, and 19% were mixed (combining two or more colors). That means the majority of lesions did not match the textbook “black spot” description most people picture.
Lesion color reflects how old and active the tissue is. Red lesions are considered the youngest and most metabolically active. They show higher levels of a protein called IL-8, which promotes blood vessel growth and cell proliferation, essentially helping the lesion establish itself. As lesions age, they darken to black, brown, or blue as old blood breaks down and iron pigments accumulate. White lesions are the oldest stage, made up mostly of scar tissue (fibrosis) with lower levels of inflammatory markers. Think of it as a lifecycle: red and inflamed at first, darkening over time, then scarring over into pale, fibrous patches.
Superficial Lesions on the Peritoneum
The most commonly found endometriosis sits on the surface of the peritoneum, the thin lining of the pelvic cavity. These superficial lesions take several forms. The classic powder-burn spots look like tiny dark speckles scattered across the tissue, similar to grains of gunpowder embedded in skin. Red lesions can look like small flame-shaped patches, tiny blisters, or areas of reddened tissue that blend in with normal inflammation. White lesions appear as pale, thickened patches or small scarred areas that can be nearly invisible against the surrounding tissue.
Clear vesicular lesions are the hardest to spot. These are small, translucent blisters on the peritoneal surface that surgeons can easily mistake for normal tissue or light reflections. Specialized techniques, like flooding the pelvis with fluid to make the three-dimensional shape of these blisters more visible, have been developed specifically to catch them. Missing these subtle lesions is one reason some people continue to have pain after surgery.
Ovarian Endometriomas (Chocolate Cysts)
When endometriosis involves the ovary, it often forms a cyst called an endometrioma. These are commonly called “chocolate cysts” because they’re filled with thick, dark brown fluid, the accumulated residue of repeated menstrual-cycle bleeding trapped inside the cyst wall. The fluid has the consistency and color of melted chocolate, which is immediately recognizable when a surgeon opens one.
On ultrasound, endometriomas have a distinctive “ground-glass” appearance: a uniformly hazy, gray interior without the clear fluid you’d see in a simple cyst. They’re typically single-chambered with poor blood flow on Doppler imaging and sometimes contain a dense clot along one wall. On MRI, the old blood inside lights up brightly on certain sequences due to the breakdown products of hemoglobin, while the iron-heavy fluid creates a characteristic dark “shading” effect on other sequences. A dark rim of iron staining around the cyst wall is another telltale sign. These imaging features make endometriomas one of the more reliably identifiable forms of the disease without surgery.
Deep Infiltrating Nodules
Deep infiltrating endometriosis is defined as tissue that burrows more than 5 millimeters below the peritoneal surface. Rather than flat spots, these lesions form firm, irregular nodules or plaques that can feel almost as dense as muscle. They commonly appear on the ligaments behind the uterus, on the bowel wall, or in the space between the uterus and rectum.
These nodules often have spiky, star-shaped (stellate) edges where fibrous tissue radiates outward, pulling on and distorting surrounding structures. Over time, the fibrosis and adhesions they generate can cause visible changes: the bowel may become kinked or twisted, the ligaments behind the uterus can become thickened and lumpy, and the natural space between the uterus and rectum can become partially or completely sealed shut. On MRI, deep nodules appear as dark, muscle-like masses with occasional bright spots where active bleeding has occurred.
Lesions in Unusual Locations
Endometriosis occasionally appears outside the pelvis, and these lesions have their own distinctive look. Umbilical endometriosis shows up as a small bluish-pink mass in or near the belly button, ranging from a few millimeters to as large as 6 centimeters. It tends to swell and become painful before menstruation and can actually bleed through the skin during a period, a phenomenon sometimes called a “menstruating tumor.” On ultrasound, it appears as a solid or partially cystic nodule with irregular margins sitting in the umbilical scar.
Diaphragm endometriosis, found on the thin muscle separating the chest and abdominal cavities, typically appears as small, dark lesions visible during laparoscopy or as tiny hypoechoic (dark) spots on ultrasound.
What Happens Under the Microscope
Regardless of what a lesion looks like to the naked eye, the defining feature under the microscope is the same: endometrial glands and surrounding stromal cells growing where they don’t belong. The glands are usually single-layered tubes that still function like normal uterine lining, responding to hormonal cycles. Around them sit small, round stromal cells alongside inflammatory immune cells and newly formed blood vessels that keep the tissue alive.
In chocolate cysts, pathologists also find macrophages (immune cells) stuffed with hemosiderin, an iron-storage compound left over from digesting old blood. This microscopic confirmation matters because visual identification alone is not especially reliable. Studies comparing what surgeons see to what biopsies confirm have found a positive predictive value of only about 45%, meaning that fewer than half of spots that look like endometriosis during surgery are actually confirmed as endometriosis under the microscope. Sensitivity is high at 97%, so true lesions are rarely missed visually, but the false-positive rate reinforces why biopsy and histological confirmation remain the standard for a definitive diagnosis.
Why Appearance Varies So Much
The wide range of appearances comes down to the stage of the lesion, its depth, its blood supply, and how much scarring has developed. A newly established implant with active blood vessel growth looks red and vascular. One that has been cycling through repeated rounds of bleeding and healing accumulates dark iron pigments. One that has burned out and scarred over turns white and fibrous. A deep nodule that has triggered an intense tissue response becomes a dense, irregular mass embedded in surrounding organs.
This variability is precisely why endometriosis takes an average of 7 to 10 years to diagnose. Surgeons trained to look only for the classic black powder-burn spots can walk right past the red, clear, or white lesions that make up the majority of the disease. Awareness that endometriosis comes in many visual forms, not just the textbook version, is one of the most important factors in getting an accurate surgical diagnosis.