If giving away riches becomes a way of life for the wealthy, Mrs. James Stewart Hooker will have helped set the trend. Last month she handed a seventy-five-carat emerald to Smithsonian Secretary S. Dillon Ripley and said she won't even deduct if from her taxes.

The half-million-dollar bauble, which once adorned the belt buckle of a Turkish sultan, was purchased by Hooker fifteen years ago to wear as a brooch (surrounded by diamonds) on her suitcoat. "If you love something so much, why shouldn't you have it?" After the death of her second husband, however, she placed it in a vault for five years. Now, she says, she has no sentimental feeling for it.

Deplaning from a Concorde at Dulles recently, she decided now was as good a time as any to donate the bauble to the public. Other Washington institutions to which she has donated funds: the White House, whose Blue Room got a $250,000 refurbishing from Hooker, and the State Department's diplomatic reception rooms.

Her siblings work hard to keep up with her largese. Her sister, Enid Haupt, bought the land below Mt. Vernon and gave it to the American Horticultural Society. Her brother, the former ambassador to England, Walter Annenberg, gave $100,000 toward renovating the Thomas Jefferson Room at the State Department.

According to a Smithsonian spokesman, all "important" stones are named so the Hope Diamond now has a new roommate, tentatively called the Hooker Emerald.