In analytical chemistry a standard is a preparation containing a known concentration of a specified element or substance. Simple standards will be a dilution of the element or substance in a neutral solvent. However, most natural samples contain a huge range of different substances, and if you are measuring elemental concentration, they may be in a different compound from the one you are using in your standard. This can cause inaccuracies, and many samples are specifically designed to be as close to the reality as possible.
There are also certified reference materials available which contain independently verified concentrations of elements available in different matrices (a matrix is the word used to describe the material of the sample, for example blood).