Amethyst because that is her favourite stone, and it is the stone for Aquarius as well.
This is the prayer that is going with the pendant..

I said a prayer for you today.....
And I know that God must have heard,
I felt the answer in my heart
Although He spoke no word,
I didn't ask for wealth or fame,
I knew you wouldn't mind,
I asked Him to send treasures
Of a far more lasting kind.
I asked that He be near you
At the start of each new day,
To grant you health and Blessings,
And friends to share your way,
I asked for happiness for you
In all things great and small,
But it was for His loving care
I prayed the most of all.
" Happy Birthday Mum"