For a down and dirty, quicky type ID on a white stone, I'd head for
the nearest pawnbroker. Odds are they'll be glad to test it with
their electronic diamond tester to let you know if it is, in fact, a
diamond. If its not a diamond you're probably looking at a CZ or a
white sapphire. Unfortunately most pawnbrokers won't have the
equipment to be able to spot the difference between those two.
If it is truly THE Avon Co. jewelry, (of perfume fame), my money is
on CZ, and I'd want to double check to see if it really is karat
gold and not some sort of plate. Your pawnbroker should have acid
on hand and can easily check that for you too. I've seen VERY
little karat gold Avon stuff.
Hoping for you that they're HUGE diamonds!!
(who has been a pawnbroker for the last 14 years)