Which fruit is used in the brewing of Belgian beer, Kriek lambic?

Kriek lambic is a style of Belgian beer, made by fermenting lambic with sour Morello cherries. Traditionally "Schaarbeekse krieken" (a rare Belgian Morello variety) from the area around Brussels are used. As the Schaarbeek type cherries have become more difficult to find, some brewers have replaced these (partly or completely) with other varieties of sour cherries, sometimes imported.