Onyx was very popular with the ancient Europeans. Onyx is an opaque colored variety of chalcedony. In ancient times, almost all colors of chalcedony from white to dark brown and black were called onyx. Today Black onyx is heat treated and colored to increase it's depth of color. These types of treatments have been conducted for many years and don't reflect on the current value of Black Onyx, which is mostly a commercial stone.
This necklace is 16" the small links are 1" by 1/2" The large links are 1-1/4" by 3/4" The stones are a mix of 4-5 & 6 mm the pear shape stones are 9mm by 6mm This necklace has a simple curb link chain. So a chain extension can be used for more length.