I found this info on platinum and platinum alloys on whiteflash.com
Platinum is a versatile, eternal metal. It is naturally white and does not fade or tarnish. It is ideal for those with sensitive skin because it is hypoallergenic. It is the safest metal for durably setting any kind of gemstone. Platinum is very dense and malleable, giving it a unique quality. When platinum is scratched very little of the volume is lost; most of the metal is merely displaced and can be re-polished with relatively little loss of metal. As platinum is worn it develops a patina – a subtle texture caused by tiny scratches. Many people like this look, akin to a pair of broken in jeans. It can be always be polished to return the bright shine, and can be polished multiple times without noticeably wearing down. Other metals lose more material over time. Gold prongs wear out and become brittle, and gold rings get thinner with wear. Platinum prongs will bend, but rarely break, and are excellent at resisting wear. This is why the somewhat counter-intuitive statement is true – platinum is softer yet more durable than gold alloys.
Pt900/Ir (900 parts platinum, 100 parts Iridium) is a good hard alloy. A great compromise between relative hardness for ease of polish, excellent white color, and good malleability. It is excellent for both casting and handmade work. Less pressure is required to set gemstones than with harder alloys. It is resistant to scratching & bending and over time is very resistant to signs of wear.
Pt950/Ir (950 parts platinum, 50 parts Iridium) is a good medium-hard alloy which is malleable and the most popular platinum alloy. It’s also the whitest and softest. Good for casting and excellent for handmade pieces, it is the best choice for soft or fragile gem setting. The greater softness requires a longer polishing process. It is less scratch and bend resistant than harder alloys but holds a stone better if an impact occurs; like a shock absorber. Over time it is very resistant to wear.
Pt950/Ru (950 parts platinum, 50 parts Ruthenium) is very hard. It has the highest melting temperature of all platinum alloys and is difficult to cast. Darker gray in color than platinum-iridium, it is less malleable, hard to solder and weld and hard to burnish. Bench workers find it tough on burs, files and drills. Some setters recommend it for diamonds only, since more pressure must be imposed on gemstones during the setting process. It is very resistant to scratching and bending, and thus resistant to signs of wear over time.
Pt950/Co (950 parts platinum, 50 parts Cobalt) is moderately hard. With the lowest flow point of these alloys it is good for even, dense castings of finely detailed pieces including filigree, but not as good for work by hand. Unlike other platinum alloys, this one tarnishes when heated so it needs flux and pickling after soldering just like gold. Since Cobalt is a ferrous metal, not from the platinum group, its scraps must be kept separate from other platinum scraps. It takes a fast polish but finishes darker gray than iridium alloys. It requires moderate pressure on gemstones during the setting process. Bench workers find it more “gold-like” and easy on the tools. It wears quite well over time. Platinum is a wonderful choice of metal for jewelry that is durable enough for a lifetime of wear, and suitable to pass down to future generations as well.