Opal can occur in many colors including white, colorless, pale yellow, pale red, gray, brown and black. Diffraction can cause flashes of any color of the rainbow which can be seen in opal's color play. Yellow, blue and green are most common, whereas violet, red and orange are the rarest colors seen through play of color. Opalescence is usually milky blue in color, but it can occur in rarer colors. The saturation of body color, the play of color and the transparency determine an opal's value.Opal gemstones can occur transparent to opaque. Transparent to translucent materials are typically more valuable than opaque materials. Almost all opal will have some visible internal fractures or inclusions such as patches, potch (common opal) or matrix rock inclusions. Opal tends to exhibit a waxy to resinous luster which helps accentuate play of color and opalescence. Finer stones often have a vitreous luster. Fire opal can sometimes exhibit slight color play, but it is better known for its vivid body color.