Realgar
From PyroGuide
Formula: As2S2
Synonyms Arsenic disulphide, Red arsenic, commonly known as Realgar. The name comes from the Arabic "rahj al-gahr", meaning powder of the mine. It's formula is variously quoted as AsS or As4S4. Yellow arsenic is the trisulfide (As2S3) and is known as Orpiment from the Latin "Auripigmentum" meaning yellow pigment.
Description: Powdered realgar is orange-red in colour and produced by sublimation of arsenopyrite or crushing the native red crystals. The latter method is employed on the ore coming mainly from China and the States of Utah, Wyoming etc. (and maybe this practice is a nightmare for everyone who collects minerals). Red arsenic is toxic of course and both toxicity and price practically preclude its use in present fireworks. Realgar in this respect suffers much the same fate as Paris Green. Generally mixtures employing realgar are sensitive to both shock and friction but sensitivity varies with different oxidizers. Many old-fashioned formulas included realgar as a component in various star compositions, for producing yellow smokes and noise units (together with potassium chlorate). While the small modern sound units are generally produced using the oxides of lead, copper or bismuth, it´s sometimes difficult to find replacements for star-making (e.g. Antimony trisulfide for some applications).
Hazards: Realgar can very easily cause cancer. In the long run it's more poisonous than white arsenic!
Sources: Realgar is very hard to get and/or generally very expensive. I once managed to find a commercial source but the price for one kilo exceeded $1000 (no I did not buy the stuff). Nowadays realgar can only be found in some painting supplies for the mentioned price but if anyone knows a cheap commercial source there will certainly be quite a lot of thankful people if you let them know what it is.