Let's say there's an operation one day producing items from asteroid and lunar material in orbit, and they want to ship goods to Earth. They try making cheap heat shields by processing rock and regolith material. they put the shields on really simple capsules. The processing doesn't chemically change the material, it just sinters it, or melts it and sprays it. They want to beat the price of a ship coming from Earth to collect cargo and take it to the surface.
The simple capsules ditch in the ocean using parachutes. They are just a metal skin sufficient to protect the cargo, a beacon, and parachutes. The heat shield is discarded after entry, the rest is reused. Capsules are disassembled and sent back to the orbital operation.
The operation is able to make shapes of rock particles that can be quite porous, of arbitrary size and shape. The capsule needs to survive entry, we'll say if it survives, whatever it's shipping does too. Cargo mass is 2 metric tons.
Edit I apologize for yet another edit, but the existing answer, while helpful, considered rock and regolith, not rock that's been processed, so I made edits above to be clear that this is about processed rock and regolith.
Also, I searched and actually found a study on this (which really surprised me). It's a NIAC study from 2012, lead author Michael Hogue So, it seems that sintered regolith will work for this, and the focus changes to the rest of the task of making this cheap. From page 36:
Within the scope of the testing to date, the feasibility of using extraterrestrial regoliths as the construction material for atmospheric entry heat shields has been confirmed from the results of the acetylene flame and arc jet testing. While some of the arc jet-tested samples were heavily ablated, they provided adequate low temperatures on their rear surfaces. These rear surface peak temperatures were recorded several minutes after arc jet test termination.
How should the capsule be designed to work best with this? What would be the best way to make use of such capsules to make the full cost of shipping from an operation in Earth orbit as cheap as possible?