children’s fantasy book with a green crystal

I’m trying to find a series of books I read as a kid. Here’s what I
know and the vague bits I can remember:

– They are children’s books for elementary school age kids and up.
(Definitely were shelved in the children’s section of my public
library.)
– Must have been published before 1980, although I suspect it was
sometime during the 1970s.
– Fantasy fiction. (Along the lines of “The Black Cauldron” series.)
– The story involved magic and wizards (but not sure they called
themselves “wizards”).
– The wizards could take the form of other living things (but I’m not
sure they used the term “shape shift” though).
– I think at one point a wizard stayed in the shape of an animal so
long that when he changed back to his human form his hands kept a
shadow image of the hooves or claws or whatever the animal’s feet
where like.
– I think a wizard may have even changed into a tree.
– A key element was a green crystal with some kind of magical power.
– The crystal was shattered and pieces (I think they used the word
“shards”) were scattered. I think they were scattered across time as
well as space.
– There was definitely a “grotto” because I remember having to look
that word up in the dictionary.
– And the only other clue I can think of is that I’m pretty sure the
author’s name is in the middle of the alphabet because I remember
going to the middle of the fiction section in the children’s room of
the library to find these books.

I hope someone can find the author and titles of these books. I’d love
to read them again to see if they are as good as I remember.