Yellow / Pale colored bricks - what are they

[brimbleshoes]

A couple weeks ago I saw an add for free bricks from a chimney teardown. I went and had a look, and I assumed they must be firebrick because they were yellow-ish, and from a fireplace. I've never actually seen a firebrick so I don't know how to compare it. So I'm wondering if I just have some yellow bricks, or what? They are smooth on all sides, but fairly easy to break. When broken the inside has sparsely flecked little white granules with the majority of the yellow / pale color. If I weigh the brick will that tell me anything. And if its not a firebrick, is it safe to use? I assume if its yellow and not a firebrick it has some other mineral that makes it more yellow. (Limestone, calcium carbonate, etc.).

This will be used for a trial/temporary oven (using fireclay and sand, no cement) so I can build one a few times and try to get it right. (Similar to what the MHA group does / did). I would build a dome and not a barrel vault, but the similar principle of the concrete block stand, and no cement.