Two tetrahedra can be set point for point in opposite orientations so that they interpenetrate, their eight vertices marking the corners of a cube and an octahedron occupying the space where they overlap. The figure was illustrated in Luca Pacioli's De divina proportione of 1509, with woodcuts attributed to Leonardo, and Kepler later named it the stella octangula. As mathematics it is a compound of two solids rather than a sixth regular solid, and it does not extend Euclid's list.
In modern esoteric writing this compound is called the Merkaba and described as a light-body vehicle activated by breath and rotation. The word is worth separating from the shape. Merkavah in Hebrew means chariot, and Merkavah mysticism is a documented early Jewish tradition, roughly the first millennium AD, concerned with visionary ascent to the throne-chariot of Ezekiel's opening chapter. That tradition has nothing to say about two interlocking tetrahedra. The geometric identification comes from the sacred-geometry literature of the late twentieth century, principally the Flower of Life workshops and books of Drunvalo Melchizedek in the 1990s. It is a modern teaching using an old name.
The chemistry, by contrast, needs no qualification. A carbon atom bonded to four others places them at the corners of a regular tetrahedron, because four things repelling one another equally around a centre settle into exactly that arrangement. The angle between any two bonds is about 109.47 degrees, the angle whose cosine is minus one third. Methane has this shape, and so does every carbon atom in diamond, whose hardness comes from an unbroken tetrahedral network. Silicon and oxygen build the silicate minerals from tetrahedral units in the same way, which makes the arrangement one of the commonest in the Earth's crust.
The tetrahedron is also the rigid one. A frame of four rods hinged at the corners cannot deform without a rod bending, while a cube of hinged rods folds over. Trusses and space frames exploit this. It is a genuine property of the shape and needs no dressing up.