The most valuable amethyst stones are a medium-dark tone (around 85%), highly saturated violet-purple (aka, "royal purple") color that when faceted shows reddish flashes (scintillation) under incandescent lighting. Stones should be eye-clean and exhibit even saturation. Stones with some deep blue color zoning are sometimes encountered, and also desirable when cut to show the blue to advantage (i.e., showing some dark blue, along with the reddish, scintillation through facets). As the tone gets lighter and/or the color saturation lessens, the value of amethysts declines rapidly. Treated stones are priced the same as lower-value and synthetic amethyst.
As to those that you describe as "conglomerates," I'm seeing stones with color zoning (and perhaps some inclusions). When well cut to highlight inclusions (e.g., rutile or cacoxenite) or zoning (e.g., pseudo-trapiche pattern), these can raise the value of otherwise lower-grade amethyst.