Rock Identification Novice, help!

Hey I am a rock and geode hunting enthusiast but a novice at best. I was hiking along the cliff side in Southern California and stumbled upon a strange half gray-half white (with crystals) rock. I cracked it open and have been searching for some kind of answer as to what I found ever since.

If anybody has some kind of information for me it would be greatly appreciated, i thought It was half-quartz. Maybe half shale, half quartz but no images I have found online look like my rock. I'm pretty sure it's not obsidian and I know it's not coal.

It flattened the tip of my nail when I was trying to crack it in half but the white parts break off really easily - into dust if I wasn't careful, some parts of the gray broke off into solid but kind of flat pieces.

I will update this post with pictures as soon as I figure out how to upload.