Stranger from the depths

This is a fun and exciting science fiction adventure for boys and girls. I'm one of four sisters who grew up in the 60s. We bought this book through scholastic books and we all read it many times over the years, it became a treasured family favorite. And now 50 plus years later, I'm hoping to find a copy to read to my 6 year old granddaughter. Having said that I don't really remember much about the plot beyond what follows. But please remember that it's been a very long time since I read it so I may not have details exactly right but here's what I remember of the premise....(no spoilers) While walking along a beach two children accidentally dislodge a boulder from the face of a cliff. The children climb on top of the boulder, which is now resting on the beach, and as they play they see what appears to be a small statue partially embedded in the boulder. As they try to remove it they realize that it is embedded in solid rock where the boulder used to be attached to the face of the cliff. From what they can see of it, the statue appears to be of some sort lizard man, and even stranger and more exciting, it looks like it was carved from solid diamond! The children tell a local geologist, who is a family friend of their discovery and take him to see it.. He carefully removes the little statue from the rock, examines it and then runs a series of tests to determind its age and what it is made of. The geologist then tells the children that because of where it was found, embedded in solid rock in what used to be part of the ancient seaside cliff, the geologist tells the children that the statue must be extremely old, so old in fact that it predates mankind! The rock it was embedded in wasn't just old, it was millions of years old!! Even more unbelievable, the statue was carved from some sort of synthetic diamond. He tells them that there is no known technology that can produce such a diamond OR even to carve it! This artifact is so unbelievably strange that i