|This is a class of gems that is natural but does not have any crystalline structure. They are not truly minerals, but fossils, or rocks by definition.
Amber is the name for fossilized tree resin. A predecessor to amber is "copal", which is a semi-fossilized tree resin. The amber is basically turpene free copal, the loss of turpene taking place over extended time.
Amber is usually a honey brown transparent material often with incorporated secondary material including, seeds, bark, leaves, and insects. It ha electrostatic properties and can generate a static charge when rubbed with fir.
It can be cut into faceted stones but very soft and is most often found in cabochons or irregular beads.
Natural glass, in this case called obsidian. Obsidian is quickly cooled magma that did not have the time to crystalize into a rock. It is predominantly pure silica trapped in the glass state.
The majority of obsidian is just black and transparent to translucent. There are a wide variety of "colored" obsidians which get their name from the type of color they display. Mahogany obsidian is black obsidian with swirls of brown material running throughout. The brown coloration is likely the result of trapped hematite.
Other "sheen" obsidians produce a shiller effect when cut and may break up the light into colors or swirls. These obsidians may contain tiny feldspar crystals or entrapped air bubbles depending on the type. The flow patterns trapped in the volcanic glass give shape to the light play in the final cabochon. "gold sheen", "silver sheen" and "velvet" obsidian are common names for ether colors or structures with similar characteristics.
Snow flake obsidian is black obsidian with spherulites of a white secondary mineral. The white material is cristobalite, a different phase of quartz. Cristobalite is a polymorph of quartz formed at higher temperature When slabbed the radial cristobalite balls resemble small snow flakes against the black obsidian background
Another natural occurring glass in from tektites. Tektites are now considers to be material that is returned to earth after an large impact with a meteor or other extraterrestrial objet. They are molten materials ejected at high temperature during the impact and then fall back to earth and "freeze" in glass state. From the pint of view of gemstones, moldavites (a form of green colored tektite found only in the Czech Repubic) are sometimes faceted into gemstones.
The moldavites tend to have more value as a collectors item than for their "beauty" in jewelry. They tend to be a dark green glass. Frankly I have seen faceted 7-up bottles that are far more attractive but not as rare.