'Stony faced' or 'stony-faced'.
From the Oxford Standard Dictionary Online (http://www.oxforddictionaries.com/definition/english/stony-faced):
Definition of stony-faced in English:
Showing no emotion; impassive:
a cold-blooded, stony-faced assassin
the foreign minister sat stony-faced and without applauding
However, even the worst sketches elicit a happy groan rather than stony-faced silence.
For their part, the women sat stony-faced, watching their menfolk make fools of themselves.
The campaigners, however, face a stony-faced industry with the law on its side.
From the OED (http://findwords.info/term/stony):
stony ▪ I.stony, a.
Forms: 1 stániᵹ, 3 stoni, 3–4 stani, 4–5 stany, 5–6 stonye, 6 stoany, Sc. staany, 6–7 stonie, 7 Sc. stanie, 7– stoney, 4– stony.
[OE. stániᵹ = OHG. steinag, Goth. stainah-s:—OTeut. *stainaᵹo-, -aχo-, f. *staino- stone n.: see -y. (OE. had also stǽniᵹ:—OTeut. type *stainīᵹo-.)]
- Combinations, etc. a. In advb. comb. with adjs., as stony-blind (= stone-blind), stony-pitiless. b. Parasynthetic formations, as stony-eyed, stony-faced, stony-jointed, stony-toed, stony-winged adjs. c. Special comb. and collocations: stony bone (tr. med.L. os petrosum: see petrosal; cf. rocky a.1 3 a), the petrous portion of the temporal bone, containing the internal ear; stony-broke a. (slang) = stone-broke (stone n. 20 a); stony coal = stone-coal; stony cobbler (see quot.); stony colic, colic due to an intestinal concretion (cf. stone-colic, stone n. 20 a); stony coral = stone-coral (stone n. 20 b); stony-iron n. and a., used to designate meteorites which contain appreciable quantities of both stony material and iron; Stony Mountains, the Rocky Mountains (see rocky a.1 1 b); stony sage (see quot.).
1933 M. Arlen Man's Mortality xv. 315 Manteuffel, staring *stony-faced towards the darkness..appeared not to have heard his question. 1975 F. Bresler You & Law 81 Even in this stony-faced sector of the law, fairness prevails.