Occurrence Jasper is typically found in veins and cracks in volcanic rocks, often together with chalcedony and agate. I havent seen any geodes filled with jasper that resemble agate geodes, so the formation of jasper seems to be restricted to veins and cracks percolated by aqueous solutions. Veins of jasper occasionally occur in igneous rocks (for example, red and yellow jasper in a granite in the Erzgebirge, Saxony, Germany). Jasper is the name of dense and opaque varieties of microcrystalline quartz. Jasper is not really a mineral in the strict sense, but a mixture of different types of microcrystalline quartz with impurities of other minerals, and is called a textural variety of quartz. Jasper of homogeneous color looks a bit like a colored, opaque flint, and shares many of its physical properties, but it forms in different environments. Multicolored jasper makes for an interesting ornamental stone, and red jasper is cut as a gemstone. Specific Properties What clearly distinguishes jasper from other valuable cryptocrystalline varieties like agate, chrysoprase, sard, or carnelian, is its opacity. Only thin chips of jasper are translucent. It might, for example, be of the same color as a carnelian, but the latter is translucent. Heliotrope is opaque and thus considered a variety of jasper (see below). The high content of embedded iron compounds occasionally causes the streak to be slightly colored, which is very different from all other quartz varieties. Most of the times the streak will be colorless or white, though. Jasper can have a very homogeneous structure and be evenly colored, and is then suitable for lapidary works. Often it is unevenly colored, either irregularly banded or patchy. Some jasper varieties are valued for their peculiar color patterns.
Posted on: Mon, 06 Oct 2014 23:53:06 +0000