80s (or earlier) book with children who have magical powers imbued by wearing stones around their neck

Question

I believe I first read this in the mid/late 80s. Here what's I can recall:

• It involved a boy and a girl - I believe 7-10 years old - I don't recall if they were friends or brother and sister
• They were given a "stone" on a string necklace that enabled magic and protection of some sort from some middle-aged/old-aged guy that lived in town
• One plot is where an evil "magician" of some sort comes to their school and turns the assembly into an assortment of snails/other creatures, but their "stones" protect them from the magic
• I vaguely recall that they were both taught transformation "spells" and could turn themselves into snails or something
• At the end of the book, I believe the boy is also able to control the weather - he makes it rain to fill up a water barrel (or maybe he stopped it raining - something like that).

It's a book that my nephew would thoroughly enjoy and I would like to get for him, however, the above is pretty much all I can remember these days - I'd recognise it if I see it though...

I remember taking this out on a "child's" library card mid 80's - not sure if that helps though. Does mean the book will have been published before then. Wishing my memory was a bit better.

Answer 1

Is it possible that Tirissa and the Necklace of Nulidor (2009) by Willow is your book?

From Amazon:

One day twelve-year-old Tirissa discovers that everyone in her village is under a spell. Everyone but her! Then she sees a mysterious stranger change into a huge bird, a bird with a beak like a sword. Did he cast the spell? Desperate to find someone who can break it, she flees, leaving her village behind. An old herbwoman tells her to seek help from a wizard who lives far away, and her journey takes her across the Three Kingdoms. Along the way she's joined by a kindly troll and a short, fat palace guard. They are pursued by the twin princes of Kellayne, the best hunters in the Blue River Kingdom, as well as by the huge, dangerous bird. Meanwhile, an evil wizard watches Tirissa and her friends in his magic mirror and plans a second spell that will kill everyone in the Three Kingdoms.

Answer 2

Was it A Necklace of Raindrops: and Other Stories (2001), by Joan Aiken?

Amazon’s summary says,

Here are eight gloriously imaginative stories for eight satisfying sessions of bedtime reading. There’s a flying apple pie, a cat that’s bigger than an elephant, a house that lays an egg, storybook animals that leap out of their books at night, and a wealth of other wonderful characters and ideas, all with the colorful, dreamlike quality of the very best fairy tales.

Answer 3

Under The Mountain?

My first thought was the book, mini series & movie from New Zealand called "Under the Mountain" from 1979.

It does not have stones on a necklace, but does involve stones that the children put magic into to defeat slug-like creatures.

The show focuses on twins Rachel and Theo Matheson. While on school summer holidays in Auckland, they are contacted by a man named Mr. Jones, who had met them briefly eight years earlier. This time, Mr. Jones reveals his true identity and mission. He is an alien, sent from another world in a battle against another race of aliens. These latter creatures were a family of slimy, slug-like beasts who could take on human form. Led by the evil Mr. Wilberforce, the slug monsters were now bent on destroying Earth and only the twins' emerging psychic abilities could turn them back.

In the final episode of the series, the twins are each required to throw a stone and focus their psychic energy into the stone to create a red and blue bridge-like construct that will defeat the Wilberforces.

I know these quotes are from the miniseries, but would be close to the book also.

Unfortunately much of this does not fit your description.