Buna-N. Also known as Nitrile,Hycar,Paracril, NBR. This is the most commonly used and because production runs are the largest, it is the least expensive, as an o-ring. It is a co-polymer of Butadiene and Acylonitrile. The butadiene is a rubber and provides the physical properties, the acrylonitrile is a plastic and provides the chemical resistance. There are hundreds of compounds available where you vary the ratio of each and effect the desired combination of properties. It is basically used to seal petroleum based fluids from -50°F thru +275°F. Hardness range of 40 thru 95A

Ethylene-Propylene- Also known as EP,EPDM,Nordel. This is used in many applications involving nonpetroleum fluids.
It has excellent ozone& steam resistance and is used with brake fluids and nonflammable "safety" hydraulic fluids up thru 300°F (400°F in steam). Steam sterilizers are a popular application. Hardness range of 50 thru 80A

Silicone- The only inorganic rubber and thus the best aging and nonleaching characteristics.The broadest temperature range,going from -100°F thru +550°F, but relatively poor physical properties and unexceptional chemical compatibility. In medical applications it is cured with a platinum cure system which results in a complete cross linkage in the molecular chain thus there is zero leaching to contaminate any system. Hardness 25 thru 80

Fluorosilicone- Take Silicone and fluorinate it and you have a material with the combination of silicone's temperature range and Buna-N's chemical compatibility. Identified by a blue color but very expensive  Hardness 40 thru 80

Viton- FKM or Fluorel or Technoflon- Expensive material that is very dense, excellent in vacuum, good acid resistance, oil resistance. Not good at low temp 0° but good to 450°F, There are a range now of specialty Vitons to address Niche applications where the standard viton is not recommended, but they are expensive. Hardness 55 thru 90

Neoprene- Although this is a very popular material for gaskets,hoses and tubings, it is not as popular for o-rings. o-rings are usually for specific applications and Neoprene is the "second" best choice for most applications. It is recommended for Freon service and usually applications where there are a variety of media/conditions involved. hardness 40-80

Teflon- This is not molded, it is machined from bar stock. Terrific chemical compatibility but not a very good seal because it is not compressible and has no resilience, but if it is being used as a static seal it is fine until movement occurs. There are teflon seals that are "encapsulated", that is FEP Teflon tubing with a silicone or viton core, fused into an endless o-ring.  The rubber core acts as a spring so that in effect you have a seal with the properties of teflon and some resilience provided by the rubber. People make the mistake of thinking that the core has something to do with the service, but in fact, if the core material is exposed to the media, the seal has already failed. Silicone and or viton are not chosen because of the seal service, they are chosen because they will withstand the temperature in the FEP teflon fusing process. These seals still have relativly poor compression set and are not recommended in dynamic applications

Perfluoroelastomer- Kalrez, Simriz. These are extremely expensive materials that in simple terms are elastomeric teflons. Outstanding chemical resistance and temperature range up to 625°. Chemical resistance of teflon with compression set/resilience of rubber, but again, because of the expense should only be considered when there is a large expense associated with down time. example, if a machine has to be shut down for seal replacement 3 times a year, and the customer is losing $10,000 per hour in lost production while it is down, and going to a perfluoroelastomer will result in seal shutdown of twice a year, it is cost effective. 60-95 hardness

Aflas- This is similar to Viton in application with superior performance is high pressure steam and oil well service. Some other minor variations in application from Viton. Should be considered in applications where viton is not providing optimum service. 80 durometer

Hypalon- Very similar properties to Neoprene with a bit better Acid resistance. Also, has the best color retention of the synthetics outside of silicone 60-80 hardness. There are also other materials such as SBR,Natural Rubber, Thiokol, Hydrin, Butyl, Norbornene & HNBR which although are not common to most O-Ring applications, do have specialty niche applications that might best be served with one of these materials. Butyl, for example is very impermeable and may replace Viton in vacuum applications where seal cost is a consideration.  50-80 hardness.

Colors- At one time an effort was made to color code compounds so that seals may be more easily identified in the field. This effort was not successful , but there was some changes made and there are several OEMs still using this coding.

• Green- A European standard that identifies Viton, so the ring would most likely be a metric size
• Green-An American standard that identifies HNBR, for Freon service
• Brown- An American standard that identifies Viton
• Purple- An American standard that identifies Ethylene Propylene
• Rust/Orange- Pretty much universal for Silicone
• Blue- Pretty much universal for Fluorosilicone
• China Red-An American standard that identifies Neoprene
• Gray-An American standard that identifies Buna-N
• Clear- This indicates a very pure, probably medical grade, silicone

For the most part however, with the exception of Silicones, synthetic rubber is not black at random. The optimum physical properties are achieved by reinforcing fillers of which carbon black is the best of those materials in optimizing properties.