You did not include a photo-selfi of your Iris lesion, so it's hard to answer. (At least one other person included a really good, almost professional grade photo of their Iris that they said they took with their phone. They told me how they did it but I haven't mastered it yet). I'm not trying to be critical, it can be very hard to be sure if a lesion is worrisome or not even with a direct exam, and you haven't even given us a picture to look at.

The information that could be important in distinguishing a benign lesion of the Iris from a non-benign one would be:

1. How long has it been there? Look at old facial photos. Get records of old eye exams. Ask siblings, parents, old girlfriends/boyfriends- anyone who may have looked closely at your eye. Was it there for a long time , or has it recently appeared?
2. Is it changing or growing?
3. Is it shedding pigment? (need a slit lamp and gonioscopy for this*)
4. Is the vision changing?
5. Is the eye pressure up (again, eye exam)
6. Have you ever had a melanoma elsewhere?
7. Breast, lung, colon, and renal cancers have all been documented as metastasizing to the Iris. This would be very rare. They would also be less likely to be darkly pigmented.
8. The most likely, by far, pigmented lesion of the Iris would be a benign Iris Nevus.
9. Much less common (rare) would be Lisch nodules (NF1), Brushfield spots (Down syndrome) , Koeppe’s and Busacca nodules (Sarcoidosis, iritis), granulomas (injury), and some other rare stuff that I can't think of during lunch.

So, see your Ophthalmologist and get it checked out. But you already knew that, right?