Intractable secondary glaucoma is seen in –
Diffuse iris melanoma
Nodular iris melanoma
Melanocytic deposits in anterior part of iris
Melanocyte proliferation in posterior uveal tissue
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.