It is explained in the first book, Altered Carbon, and again in the later two, that stack technology was reverse-engineered by humans (although not stated by who) after similar technology was discovered in Mars, left behind by the Martians.
From descriptions in Altered Carbon, and again in Broken Angels, we know that stacks are about the size of a cigarette butt and fitted to most humans after they are born.
Stacks themselves are referred to as 'altered carbon' in the first book by both Kovachs and Kadmin, although this could be a poetic reference as opposed to an actual description of the technology.
When describing Kadmin, Kovach says:
In an earlier age, he would have been a shaman; here, the centuries of technology had made him more. An electronic demon, a malignant spirit that dwelled in altered carbon and emerged only to possess flesh and wreak havoc (emphasis mine).
Later, Kadmin asks Ortega:
Where is the voice that said altered carbon would free us from the cells of our flesh?
In chapters 8 and 12 of Broken Angels there are several descriptions of stacks as being 'metal':
Most of it was gone, and the final pieces of vertebrae were being eaten away from the tiny metal cylinder of the cortical stack.
Later on Wardani holds the same stack 'between thumb and forefinger', giving us a vague sense of the size of the stack.
Later in Chapter 12 the sound of stacks being poured is described:
As we watched the skip tilted forward and something started to spill over the lip, cascading onto the deck and bouncing up again with a sound like hail stones. Cortical stacks.