This is from IIH:
>Furthermore, it is within the magician's capabilities to carry out other influences as well. On the astral plane the magician can condense the Earth element in his astral body and materialize to such a degree that he can appear right in front of an initiate or a non-initiate and be seen with the physical eyes, and also be heard and felt. How successful the magician will be in this respect depends upon the amount of time he has dedicated to the practice of these exercises and his ability to accumulate the Earth element in the astral body. This leads to the logical conclusion that the magician is also able to perform physical tasks with his astral body. Here you will find the correct explanation for producing phenomena in the manner initiates understand it, such as causing knocking sounds and carrying out various tasks. No limits exist for the magician, and the area of magic he wishes to specialize in is entirely his choice. He also knows how to accomplish everything, for example he may wish to condense only a part of his body, such as one hand, while the other hand remains in the astral state. Should he be able to accelerate the vibrations of the electrons of an object with the aid of his imagination, he will also be able to make the object disappear right in front of other people and transfer it to the astral sphere, all of course in accordance with his powers and development. Under these circumstances the physical objects are no longer subject to physical laws; they become astral. It is a simple task for the magician to carry such objects with the help of his astral body even over great distances and then restore them to their original form.

Can I take this to mean that the primary deciding factor of whether an object exists in the astral or physical is the accumulation of the Earth element? This seems like it makes sense but it doesn't explain why Gnomes, being Earth elementals, exist in the astral density rather than the physical.