Milk glass was invented not more than a century ago in 1845, only 20 years after when glass had began to be an art. At that time the coloring of glass was more of a trade secret, but the colors opal (milk glass), amethyst, yellow, blue and reds were the colors that one would see in glass. However color glass was was found to be rare and scarce - red is the rarest of these colors
Milk glass is said to look like porcelain white, but when held up to light it has a blue tint which resembles an opal. Collectors specifically look for the milk glass that has nods on it and various popular designs including candy dishes and vases. Specifically when hunting for milk glass is making sure it looks like milk and is opaque! There are several imitation pieces floating around - a clear trademark of milk glass though is the blue tint in regular lightening and possibly an artist signature.
Many grandparents more than likely have milk glass somewhere in their home for it was a popular household furnishing and wedding gift due to the white color. The most popular is the McKee animals which are various animals (I've always seen rabbits but I heard there are others) - these pieces are all signed, and do not be fooled by pieces that look like it but maybe lacking the signature.
My grandmother (like many others) has a love for milk glass and they make very nice gifts when they can be found at an inexpensive price at a yard sale or even an upscale thrift shop.
Happy milk glass hunting!