Howlite is a calcium hydroxy-silicoborate mineral (Ca2SiB5O9(OH)5). It has a nonmetallic luster, a usually whitish color, a white streak, no cleavage, and is moderately soft (H=2.5 to 3.5). It has the ~look & ~feel of porcelain. It will polish very well and will readily accept dyes. Artificially colored howlite is often used to mimic turquoise (honestly presented dyed howlite is often called "turquentine"). Howlite is one of many minerals found in lacustrine evaporite deposits.
Howlite (polished slice, 5.4 cm across)