The chiasmata become visible during the diplotene stage of prophase I of meiosis, but the actual "crossing-overs" of genetic material are thought to occur during the previous pachytene stage.

What phase does chiasmata form?

Chiasmata is formed in the Diplotene phase of prophase 1. Note: In the prophase I of meiosis crossing over take place. The point at which the crossing over takes place is called the chiasmata.

At which stage of mitosis do you first see the chromosomes appearing?

Prophase is the first stage of mitosis. In prophase, chromosomes condense and become visible.