No. Many women prefer colored stones, such as sapphires, rubies,
and emeralds. Colored stones can be set by themselves or with
diamonds. Another possibility is to have a diamond or colored
stone solitaire, with a wedding band that fits together with it
(sometimes known as a "wrap") and contains diamonds or colored
stones.
A woman's engagement ring can be as expensive or inexpensive as
is appropriate to the finances and wishes of the couple. Some couples
elect to have a diamond-like substitute, such as a cubic zirconia.
This is not inappropriate, and there is no reason why this choice
should need to be public knowledge.