so, this is a the sphere for our bomb. (already said modern, can be made of graphite or beryllium ) so to reflect the neutrons (and reflexion will give it thousand of millions of velocity) , these are called "lenses". two types of explosives are used on the lenses, the RDX/TNT called cyclonite, outside the lenses (high velocity) ; and an explosive, inside the sphere, called Baratol (barium nitrate and TNT) , because inside the sphere we need a low velocity detonation explosive ; all of this was a coupled inside hydrogen. their nuclear bombs have a power detonation electronic system with 5 thousand volts, with 35 detonators devices. These sphere has a nucleus, called the neutron initiator, with polonium 210 and beryllium (build like yesterday picture) . So, this is called a tamper system. the lenses have this shape, as in optics to reflect light, in a convergent mode. (means, diamonds would be perfect, right? because it would generate thousand of light beams) however they are called "lenses" but only blocks of explosives, and not glass, is the shape which is in question.