Some minerals do not form crystals and some minerals are organic.
Amber is both of that : a fossil resin, which mostly forms rounded masses
and aggregates in matrix. This matric may be a bluish clay like in the
case of the classic baltic amber and it may be a sand or sandstone. All
this deposits are secondary deposits, as the amber orginates from trees,
which grew in wooden areas. So the primary deposits of amber should be
fossil forests and indeed you can find amber often in lignite seams.
The amber content of lignite seams can be very high. The Bitterfeld
lignite deposits in eastern Germany for example contained very high amounts
of amber, which where commercially exploited on its own and sold as 'original
baltic amber' in socialistic times. In fact the production of 'lignite
amber' exceeded the production of true baltic amber by far. However
the maximum size of lignite amber is usualy limited and head sized masses
are exceptional. But not so in Malaysia.
The Merit Pila lignite field in central Sarawak is the most
important coal deposit of Malaysia. The estimated reserves are at 250 millon
tonnes, whereas much of this reserve lies close to the surface and can
be mined by opencast. The lignite is of miocene age and usually forms
only relatively thin seams within sand and gravel sediments.
Frequent 'amber lines' - long and thin seams of yellow golden
amber - can be found in the exposed coal seams, which possibly form the
original floor of the miocene forests. These mostly thin, but sometimes
up to 30 cm wide lines may reach a continuous length of almost 130 m !
(SCHLEE & CHAN 1992) These immense volumes of amber were produced by
trees of the genus Dipterocarpacaea, which still grow in this area and
still produce copious amounts of resin today.
1991 a german - malaysian expedition set out to salvage the 'largest
amber piece in the world' and they succeed to delineate an amber piece
of 3,5 m length, 1,5 m width and some decimeter height. This piece was
reduced to the size of 2,3 m by 1,3 m for easier transport, but was finally
further divided into three specimen for display in the Sarawak Museum in
Kuching, the Museum of the Geological Survey of Sarawak and the Stuttgarter
Naturkundemuseum in Germany.
Update December 2006 : Some weeks ago I visited the Naturkundemuseum Stuttgart and - though it has an extremely impressive collection of fossils and extinct animals and even a spectacular amber collection - I was not able to find a single trace of the giant Merit Pila amber specimen decribed above. Maybe somewhere in the archives...?