Use of an aluminium and gold precipitate for hardening a 18 carat gold-based alloy, characterized in that said precipitate is an aluminium and gold precipitate Al 2 Au 5 for hardening a gold alloy comprising at least 75% of gold, 0.1% to 2.1% of aluminium, and the complement to 100%, comprised between 22,4% and 24,0%, of at least one additional metal, chosen for its ability to favour a stable FCC structure on the one hand, and for its ability to increase the solubility of aluminium in gold on the other hand, said use resulting from the insertion of said aluminium and gold precipitate Al 2 Au 5 into an FCC structure resulting from the solution heat treatment of said mixture comprising gold, aluminium, said at least one additional metal, and aluminium and gold precipitates, in that said insertion is realised by a method for obtaining a 18 carat gold-based alloy, with improved hardness, according to which: - a second metal is chosen for its ability to form precipitates with gold, said second metal being aluminium; - at least one additional metal is chosen for its ability to favour a stable FCC on the one hand, and for its ability to increase the high temperature solubility of said second metal in gold on the other hand; - the conditions are created for inserting precipitates of aluminium with gold into an FCC structure resulting from the solution heat treatment of a mixture of gold, aluminium and said at least one additional metal; - a mixture is prepared comprising in weight percent at least 75% of gold, 0.5% to 2.1% of said second metal, and a complement made with said at least one additional metal, aluminium and said additional metal being selected to obtain at least one said particular precipitate of aluminium with gold to form an intermetallic compound giving said alloy a hardness of more than 250HV; - the selection of said at least one selected precipitate is restricted to only the single precipitate Al 2 Au 5; - said mixture is solution heat treated by being heated to between 650°C and 700°C; - after said solution heat treatment, rapid cooling is performed in the form of a hardening; - after said rapid cooling, a tempering structuring treatment is carried out at a temperature of between 200°C and 250°C to create said selected aluminium and gold precipitate Al 2 Au 5; - said selected aluminium and gold precipitate Al 2 Au 5 is grown in a controlled manner by maintaining said tempering structuring treatment for a sufficient period of time to obtain the desired hardness; - cooling at ambient temperature is carried out, and still characterized in that said additional metal is selected from among following elements: silver, chromium, copper, iron, hafnium, manganese, niobium, palladium, platinum, vanadium. - a second metal is chosen for its ability to form precipitates with gold, said second metal being aluminium; - at least one additional metal is chosen for its ability to favour a stable FCC on the one hand, and for its ability to increase the high temperature solubility of said second metal in gold on the other hand; - the conditions are created for inserting precipitates of aluminium with gold into an FCC structure resulting from the solution heat treatment of a mixture of gold, aluminium and said at least one additional metal; - a mixture is prepared comprising in weight percent at least 75% of gold, 0.5% to 2.1% of said second metal, and a complement made with said at least one additional metal, aluminium and said additional metal being selected to obtain at least one said particular precipitate of aluminium with gold to form an intermetallic compound giving said alloy a hardness of more than 250HV; - the selection of said at least one selected precipitate is restricted to only the single precipitate Al 2 Au 5; - said mixture is solution heat treated by being heated to between 650°C and 700°C; - after said solution heat treatment, rapid cooling is performed in the form of a hardening; - after said rapid cooling, a tempering structuring treatment is carried out at a temperature of between 200°C and 250°C to create said selected aluminium and gold precipitate Al 2 Au 5; - said selected aluminium and gold precipitate Al 2 Au 5 is grown in a controlled manner by maintaining said tempering structuring treatment for a sufficient period of time to obtain the desired hardness; - cooling at ambient temperature is carried out, and still characterized in that said additional metal is selected from among following elements: silver, chromium, copper, iron, hafnium, manganese, niobium, palladium, platinum, vanadium.