I'm not convinced this was a lilac if the flowers were that big and lasted all summer.

It was definitely a lilac. Her favorite flowers were lilac & lily of the valley. I lived with her most of my childhood and know that tree well.

I agree with floral that the plant in question is unlikely to be a lilac. There is no lilac species or hybrid that holds its blooms all summer. Most will bloom early in the season for a couple of weeks at most. Even the so-called reblooming forms (like Bloomerang) are very intermittent in their flowering and the flowers on all types will dry and brown in a matter of a few weeks.

Also, the largest individual lilac flowers are far less than a half inch across - nowhere near the size of a nickel - and 7' is pretty short for a common or standard lilac. 12-15 feet is far more typical.

it is 100% a lilac. Maybe my exact measurements are off (they haven't lived in that house in 20 yrs), but it is a lilac without a doubt.

Then go buy and plant whatever lilac you find that most closely fits what you remember. There is no way we can come close to naming a variety that fits the criteria you state. They simply do not exist!

My guess is that it was something with lilac in its name because of the bloom color and not really a lilac.

tj

There are two possibilities here. Either it wasn't a true lilac, or, it was a true lilac but you've misremembered the flower size and bloom duration. There are honestly no lilacs which fit the criteria given.

I feel your search deserves a fieldtrip next spring. Rochester Lilac Festival - Rochester Events

I will also add that if it was in your Grandma's yard, I think Bloomerang would be too recent a variety. Could be wrong about that though. And I agree with the other comments--lilacs simply don't bloom all season. In fact, the bloom time is pretty short.

some other plants which may or may not have "lilac" in their common names could be vitex, weigela and philadelphus., all of which have somewhat larger flowers than syringa. only the weigela and philadelphus flowers would approach the size described.

what about hibisbus syriacus? blooms all summer but the flowers should be larger than a nickel.

You don’t say where (what zone…?) the tree was growing in. Could it have been some form of jacaranda?

OP says they're in Michigan. I think that rules out Jacaranda. 😉