If you like the look and theme of gemstones set in a row (which characterizes the mother's ring), may we suggest the three-stone diamond pendant, which is gaining popularity as a Mother's Day or birthday gift.
The diamond line bracelet, in a gold or platinum setting, is another timeless gift that's appropriate for Mother's Day. (These bracelets are especially useful if the recipient happens to be a lucky mom or grandmother, who might have more "jewels" in her family than will fit symbolically into a ring.)
If you're buying upscale jewelry for a young girl to commemorate her graduation, Communion, or other milestone, the key is to get something that won't fall apart from rough handling. Sophisticated-looking enameled gold brooches are popular with girls, and so are bangle bracelets, which are just about indestructible.