The best sand by far is silica based sand. Aquarium sand is ok but you may be creating issues if you use marine aquarium sand as it has a high limestone content you will be messing with the ph balance of your tank.
The most common constituent of sand in inland continental settings (e.g. deserts) and non-tropical coastal settings (e.g. beaches), is silica (silicon dioxide SiO2), often in the form of quartz.
Arkose is a sand or sandstone with a high content of feldspar (an Aluminum Silicate), usually derived from the weathering and erosion of nearby granite. "Aluminum is a protoplasmic poison and a pernicious and persistent neurotoxin". No living systems use aluminium as part of a biochemical process. It can have devastating accumulative effects in humans though I am unsure what it would do to an axolotl
Much of the fine white sand found in coral reef settings is ground-up coral (limestone) that has passed through the digestion system of the parrot fish.
Some places have sands that contain magnetite, clays, chlorite, glauconite, or gypsum. Sands rich in magnetite are dark to black in colour. The chlorite - glauconite bearing sands are typically a green colour. The gypsum sand dunes of the White Sands National Monument in New Mexico are famous for their white colour.
All commercial sand is washed as it is part of the extraction and preparation process. The problem with builders sand is that there are many different grades depending on the application ie: masonry, bricklaying, cement etc. It is also not as uniform as say pool filter sand which is why it appears to be unwashed.
Have I sold you on no substrate yet? But seriously if you stick to playsand or filter sand (which incidently can come in a variety of colours) you can't go to far wrong.