Need Help from a Geologist?

It's better to have you try to look at it and get some more detail. The macroscopic characters of rocks, those visible without a microscope, are quite varied and difficult to describe accurately and fully.

With a small bottle of acid to test for carbonate of lime, a knife to ascertain the hardness of rocks and minerals, and a pocket lens to magnify their structure, we have a better shot at identifying a rock in the field. A simple magnifying glass (different from a pocket lens) and magnet are also important. It is easy to see that a sandstone or grit consists of more or less rounded, waterworn sand-grains and if it contains dull, weathered particles of felspar, shining scales of mica or small crystals of calcite these also rarely escape observation. Shales and clay rocks generally are soft, fine grained, often laminated (layered) and sometimes contain minute organisms or fragments of plants. Limestones are easily marked with a knife-blade, sizzle with weak cold acid and often contain entire or broken shells or other fossils. The crystalline nature of a granite or basalt is obvious at a glance, and while the granite(s) contains white or pink felspar, clear vitreous quartz and glancing flakes of mica, the basalt will show yellow-green olivine, black augite and gray stratiated plagioclase.

Your description is very strange. Okay, it's a fairly large chunk of rock for hand-specimen. Light tan and smooth could be chert - but talc is what and slippery, not smooth (there's a distinct difference - talc is like soap... in fact, many people used it as soap before the Dial bar came along). But heavily fissured with open pores/vesicles is very strange. Especially if you think these holes had been filled with a mineral that has since been dissolved away - crystals only get that large when a granite (or similar) cools very slowly underground. And... you don't get inclusions like that in a chert.

It doesn't sound anywhere close to being sedimentary, which is what many Texas rocks are.

But... and don't quote me on this, because I can't actually see your rock, or test it.. and I don't even know which part of Texas it's from... but my vague guess is a big lump of chert -

Chert (microcrystalline quartz) (SiO2) includes chalcedony, agate, jasper, and flint. Chert and flint are so similar that there is no sharp distinction between them. Dark-colored nodules are called flint, and the light-colored variety is called chert. The variation may be due to the inclusion of variable amounts of organic matter. Other chert colors can include pink, brown, and purple. Chert has a conchoidal fracture, a hardness of around 7, a dull luster, and a colorless streak.

The irregular masses of chert that are common in Texas limestones may have been formed by silica-rich groundwater passing through the sediments before they were lithified (formed into rocks). These irregular masses, known as nodules, often have exotic shapes uninfluenced by the textures in the host limestone. That might explain the strange cracks and pockets that you're seeing.

Best guess... hope you find out!