There are no two crystals alike. Take your crystals and really look at them. My little sample that I have, picture attached, explain the complex uniqueness of each. IN the picture is a smokey quartz crystal, size about double fisted. Yes a few of you have seen it, held it and all wanted to take it home. OK so what it is a double terminated smokey quartz. Problem is that for its size it is pretty rare. Slightly ingrown (twinning) is a small amethyst crystal, can be seen on the lower left.
Now you have a crystal, smokey quartz, double terminated with a twinning amethyst crystal attached. Cool so what, you have to remember that you have to start blending the characteristics of the crystal. Playing with the crystal in sunshine indicates a high refraction making it a multiple rainbow crystal. Furthermore, the small amethyst crystal does have two small parrallellogram facets on the head, making it a double window. But that double window looks into the main crystal. The striantions which is triangular receiding into the main crystal, makes it a library, or recording record keeper.
And the coup de grace, it is also a twin enhydro. One bubble on the flat side and one bubble on the one top facet. Now just go and try to put all these different types of crystal into one coherent crystal. THat is why this crystal is so strong, unique, and I have to say she has a twin sister.