I am writing a setting for a role-playing game. In it an alchemist has finally found a way to create gold from lesser metals.
Like his former historical models the alchemist is so jubilant over his discovery that he seems to ignore that his "gold"...well, it really looks like gold and seems to be metallic, but some things like density, conductivity, malleability are absent. In short: It is not gold.
What I need is a substance which could have been mixed by an alchemist and is as gold-like as possible. May contain heavy metals or poisonous substances, the alchemist may be...a little affected.
The well-known pyrite is the soft-option, I do not find it particularly compelling as gold ersatz.
Some additions to the proposed solutions
We are talking about a fantasy world with mediaeval technology level. All metals with a comparable density level are out of the question: tantalum, tungsten und uranium are out of the question, their necessary production conditions make them nearly impossible to produce. Platin metals which have an even higher density are even rarer than gold.
For the same reason radioactive decay or nano clusters are out of the question. It takes high technology levels to produce that result and the cost is astronomical.
Mixing gold in small parts does not work. In fact the experience with gold alloys shows that at least 75% volume per cent is necessary to get the given color.
Best solutions so far are brass alloys:
Nordic gold: Looks definitely like gold and is a metal.
Composition: 89% copper, 5% aluminium, 5% zinc, and 1% tin.
Problem: Aluminium cannot be produced because it needs high-powered electrolysis and rhyolite which is not available at this time.
Another very gold-like metallic alloy suggested by a friend is gold tombac or red arsenic tombac.
Gold tombac >85% copper and zinc
Red arsenic tombac: 98% copper and 2% arsenic
Lead iodine: Looks like gold powder.
For those who would like to know how characters can test gold: The metal workers and also the alchemists knew since ancient times that heating and melting gold was a very good method to test purity because normal alloys are separating. Another old method is the stroke test: Move both gold and the unknown metal over a stone and compare the results. Sounds crude, but is very, very good to detect fake gold. Density measurement is another option: Get real gold and a probe of the unknown metal with the same weight. Fill a container with water and drop the gold into it, so that the gold is completely submerged; mark the water level. If the unknown metal is gold, it will have the same level, if not, the water stands perceptibly higher.
ANOTHER CLARIFICATION: Many people have wondered or could not believe that alchemists which are capable to create new stuff are not able to admit that it is not gold. It sounds strange, but it is in fact true.
In "Alchemy: The Philosophers's Stone" from Allison Coudert it is explicitly mentioned in the chapter about transmutation that alchemists believed in several forms of gold. Geber said that gold is created from most subtle mercury substance and some pure, red, fixed sulphur substance which gives gold its color. Like the sulphur has different tones of red, so must have gold different shades of yellow. Chen Yin, a chinese alchemist, mentioned that the gold created from transmutation incarnates so many chemical ingredients that is superior than natural gold. Some of them drank actually tin (IV) sulfide as "liquid gold".
It is also mentioned that the alchemists stayed away from practical metal workers because they felt grandiose. Allowing to test their gold sounded like a sacrilege. There were also many, many stories about witnessed transmutations (Johann Konrad Richthausen, Wenzel Seiler und Nicolas Flamel. Yes, the guy from Harry Potter did exist in reality). So alchemists were easy prey to delusions of grandeur, their environment (mercury etc.) did not help exactly.