Intractable secondary glaucoma is seen in –
Iris melanomas may be of two types :-

Circumscribed/ Nodular: - It is the most common type which presents as a pigmented spot on the iris. Glaucoma is less common and usually treatable.
Diffuse iris melanoma: - Present as a unilateral dark iris (hyperchromic hetrochromia). The tumor has diffusely infiltration pattern of growth and invades the intertrabecular spaces, and plugs the drainage mechanism. This results in intractable secondary glaucoma.
Diffuse iris melanoma
Nodular iris melanoma
Melanocytic deposits in anterior part of iris
Melanocyte proliferation in posterior uveal tissue