Purity of bloodline would be nice, but purity in general works.
There are many symbols. A number of symbols indicate pure Kosher foods such as "K" in a circle, star, or triangle, "U" in a circle, and a few others. Chemicals pure enough for predictable reactions are labeled GR for Guaranteed Reagent grade though some lab work requires even higher purity levels. USP indicates medications, supplements, and food additives that meet high standards for uniform purity and strength. High purity (99.9%) gold is stamped "24 K" or "24 Kt" for 24 carat. Since John 3:16 is considered by many to be the purest and most succinct expression of the Christian faith sometimes its chapter and verse "3:16" is used elsewhere to indicate other pure and succinct expressions. The already mentioned Lotus and white roses are symbols of purity. White wedding dresses are symbols of virginal purity of the bride. Purity rings are worn by unmarried men and woman who vowed to remain sexually pure (virgin) until marriage.
Were original use of swastika's meant for things like purity, like how Buddhists used them, I think they are set at different angle to the how the nazis used them (like they straight, not turned)
In Buddhist symbolism, the lotus represents purity of the body, speech, and ... birth) is added to synonyms of water to derive some synonyms for the lotus.
White. As in a white dress. Water, clear clean water. Just things that came to mind, not a real good answer, but throwing it out there.
When I think of purity, I think of doves.
the colour white, children.