I am not an avid collector but I found a rock that was really out of place and have had it for 40 years now. So...basically, it's about time to figure this out. I found it in a field on the east side of San Antonio, Tx. It definitely did not belong where I found it. It appears to be lava but I can't rule out a meteor. I would really appreciate any help I can get.This site only let me upload one pic at this time. I have more.
Slag is a byproduct of smelting and contain a myriad of materials but is most commonly high in silicates hence the porcelain feel. Different textures are produced depending on the composition, initial temprature, and even how fast it is poured out or how fast it cools. The same process that go into different laval textures go into making slag hence the similarity in appearance. Your piece looks like it cooled slowly making pit slag likely, where large quantities are dumped all at once.
I would suggest that the rock is a lava rock "Basalt" from a Pahoehoe flow. I have collected similar rocks in California, Arizona, and New Mexico, but I haven't managed to get as far East as Texas. One area I have collected similar rocks from is the Pisgah flows in California. Pisgah Crater I have also seen slag that is similar looking but slag almost always has some iridescence, and has some differentiation of crystal size or texture over short distances within a single sample. Lava is also used extensively for road ballast in areas where it is available.
I will try and search my yard for an example piece I can use to post a picture. This type of lava does has a distinctive sharp "ting" type noise when struck lightly with a rock hammer. In the meantime this site has an image of Pahoehoe lava, similar lavas from the Western states are normally duller black with some reddish interiors through there is a lot of variability. Washington University Lava