What mineral/crystal is this?

Question

I found a rock in a parking lot that is a dirty white color flattened oval shape, 5" at it's longest point, 3" across (on the flat part) and and was an inch at the end and two inches in the middle (rough measurements). I went home and hammered it for a little bit until it broke. While the outside was slightly rough to the touch (chalk-like texture) the inside has a glass like texture although it was not a clean break. The inside is pure white, reflective, a little flaky where it is thin, and has no visible crystal structure. The only other thing worth mentioning is that the outside sparked when the hammer head (made of iron) hit it.

Update:

@ Tom all the nail did was leave a black line on the rock that i can rub off. There is no discernible scratch.

Tom G · Accepted Answer

It sounds like you found some chert; try scratching it with a nail and see what happens (chert has a mohs hardness value of 7 and steel around 5.5). Also does it have "colloidal fractures" like a piece of broken glass? Something like this:
http://geology.com/minerals/photos/quartz-conchoidal-482.jpg 
or this:
http://web.eps.utk.edu/courses/rock/images/Chert.JPG

If it is, the outer layer is usually white due to removal of the inclusions that that give chert (microcrystaline quartz) its color and is called a patina.

Edit: Yup sounds like a piece of chert then, the shape of the stone itself is most likely the result of the way it was transported; likely it was weathered out as a chert nodule and was transported by a stream (as opposed to being from a beach) till it obtained its current morphology (shape). I see the things all the time in rock gardens.

Trending News

What mineral/crystal is this?

1 Answer