I am obviously not an expert in minerals or their values, but I can share with you my perspective on value and buying minerals.
Some of the factors that influence Mineral Values
1. Aesthetics – it is all about being pretty. Beauty; it is subjective but yet over and over different people will pick one item consistently over another as more attractive. It is about the interplay of shape, texture and color.
2. Form – crystal forms of a mineral are considered more valuable than non-crystal forms of minerals. I find crystal forms in their matrix more pleasing than crystals not in a matrix.
3. Crystal Size – All things being equal, the larger the crystal the more valuable it is.
4. Specimen Size - Large cabinet size minerals are generally more valuable than their smaller counterparts; but a large mineral may be no more valuable than its value as a collection of smaller specimens. For it to demand a premium, a large size mineral must exhibit a special aesthetic quality separate from its largeness.
5. Rarity – Rarity factors include mining location, the mineral’s innate rarity (i.e. gold is rarer than quartz), its unique association with other minerals, unusual form, and other factors I am not that well versed on.
6. Perfection – The more perfect the mineral the more valuable it is. When it comes to the crystal form, chips, fractures and cloudiness will decrease a mineral’s value. Crystals that stand straight up on their matrix, have high clarity, purity of color, and have terminated ends free of defect, command the top prices.